The decision this actually comes down to
Ask what happens to the clip after it renders. If it is going into an ad set as one of many variations, most of which you expect to lose, then cost per attempt is the only number that matters and Grok Video is the answer. If it is the asset you will crop, caption, reuse across placements and spend behind for weeks, then a 1080p master is worth paying for and Veo 3.1 is the answer.
That framing beats arguing about which model is generally better, because the two are not priced as substitutes. At 14 credits per second against 31, Grok is not slightly cheaper. You can render two 10-second Grok clips and still have credits left over compared with a single 8-second Veo clip.
The trap is using the expensive model for exploration. Testing is where credits disappear fastest, and paying a premium per attempt before you know which concept works is the single easiest way to burn a plan without learning anything.
What Grok Video is: the volume tier, with a hard resolution ceiling
Grok Video is xAI's video model, and on UGC Vids AI it runs image to video: you supply a start image (an avatar, a product shot, a composite) plus a prompt describing what happens, and it renders up to 10 seconds with its own audio. It is priced in two resolution tiers, 480p and 720p, and there is no 1080p option because the model does not have one.
The price ladder is short and cheap. 480p costs 50 credits for 5 seconds and 100 for 10. 720p costs 70 credits for 5 seconds and 140 for 10. On the $49 Starter plan those work out to roughly $0.49, $0.98, $0.69 and $1.37 per clip. There is almost no reason to use the 480p tier for anything you will show a customer, but at 50 credits it is a legitimate way to sanity-check a prompt before committing.
One practical detail that saves reframing work: Grok follows the aspect ratio of the image you give it by default. Feed it a 9:16 avatar shot and you get a vertical clip without having to think about it. It also accepts square, which matters if you run 1:1 placements.
The ten-second cap is more generous than it sounds. Two seconds longer than Veo's maximum is the difference between a hook that ends abruptly and one that lands a full sentence before the cut.
What Veo 3.1 Fast is: eight seconds, 1080p included
Veo 3.1 is Google's video model, and the variant running on UGC Vids AI is Veo 3.1 Fast. Like Grok it starts from an image, takes a prompt, and generates dialogue, ambience and effects in the same pass as the picture rather than needing a separate audio step.
Its price ladder has three rungs: 120 credits for 4 seconds, 180 for 6, and 245 for 8. About $1.18, $1.76 and $2.40 respectively on Starter. The important line in that ladder is what is missing: there is no resolution premium. 1080p costs exactly the same as 720p, so if you are rendering on Veo you should always be rendering at 1080p.
Eight seconds is a real constraint, not a soft one. It is the model's native ceiling per call, so a 20-second Veo ad is three clips stitched together, with the continuity work that implies. For a hook, an eight-second single take is plenty. For a scripted product walkthrough it is not.
Veo also has the narrowest aspect support of the two: portrait or landscape only. If your campaign needs 1:1 assets, that is a point for Grok or for one of the models that accepts a square frame.
The 720p ceiling, and when it stops mattering
Grok's 720p cap is the honest reason not to use it for everything. Both TikTok and Meta re-encode what you upload, which softens the difference in feed, so a 720p vertical clip viewed on a phone is not the disaster a spec sheet makes it sound like. The problem is what you do afterwards.
Resolution is headroom for editing. Crop into a 720p frame to reframe a shot, punch in on the product, or add a zoom in post, and you are enlarging pixels you never had. Do the same on a 1080p Veo master and the crop still holds up. If the clip is going to be cut, reframed or reused across placements, start with the higher resolution.
So the rule is not "720p is fine" or "720p is bad". It is: 720p is fine for clips that ship as rendered, and expensive later for clips you plan to work on.
Cost math: clips per plan
The Starter plan is $49 a month for 5,000 credits, so a credit is just under a cent. At 70 credits, that plan funds about 71 five-second Grok clips at 720p. At 140 credits, about 35 ten-second Grok clips. At 245 credits, about 20 eight-second Veo clips.
Put per-second: Grok is 14 credits per second at 720p and 10 at 480p. Veo is about 31 credits per second across all three of its durations. So every Veo clip you swap for a Grok clip of the same length roughly doubles the number of attempts your plan buys.
A concrete monthly plan that uses both: 30 Grok clips at 720p for concept and hook exploration (2,100 credits), then 8 Veo renders of the concepts that survived (1,960 credits). That is under 4,100 credits, comfortably inside a Starter month, with room left for a few image generations. On the $99 Growth plan at 12,000 credits you can run that same loop nearly three times over.
Because everything on UGC Vids AI runs off one credit balance with no per-video cap, you never have to decide the split in advance. Spend the month however the tests break.
Which model for which ad job
Hook and concept exploration: Grok Video, at 720p. Cheap enough that a clip which does not work costs you almost nothing, long enough at 10 seconds to show a full beat.
Prompt sanity checks: Grok at 480p, 50 credits. Use it to confirm the model understood your scene description before you spend anything real.
Hero creative and anything you will edit: Veo 3.1, at 1080p, because 1080p is free on this model and because the clip you spend behind should be the highest resolution you can get.
Square placements: Grok, since Veo renders portrait or landscape only in this pipeline.
Clips between 8 and 10 seconds: Grok, by default. Veo simply cannot render them in one pass, and stitching two Veo clips to reach 10 seconds costs 300 credits or more and introduces a cut you may not want.
Verdict: Grok for attempts, Veo for the one that ships
Neither model is a replacement for the other, and the reason is structural rather than aesthetic. Grok Video buys you more attempts per dollar and two extra seconds of runtime, and charges for it with a hard 720p ceiling. Veo 3.1 buys you a 1080p master at no resolution premium, and charges for it with an 8-second cap and roughly double the per-second cost.
If you only ever run one of them, run Grok for a month and see how many of your clips actually get re-edited. If most ship as rendered, the 720p ceiling costs you nothing and you have doubled your testing volume. If you are constantly cropping and punching in, that is the signal to move your winners to Veo.
Both models sit in the same picker on UGC Vids AI, on the same credit balance, so you can render the same hook on each and compare the two files side by side before committing a month of budget. The $1 trial is enough to run that test. Cancel anytime.