Computer Science: Workshop paper in top conference vs. Paper in mediocre conference?
Both previous answers are misleading in this specific scenario, where the choice is between CVPR (main conference) and an ACM conference.
Both conferences require submission of a full paper! The only difference is that CVPR will decide after the review how your paper is presented, whether as long oral presentation, short oral presentation, or poster. Independent of this, all accepted CVPR papers will be published in the conference proceedings without mention of the presentation type. You cannot submit your paper to both conferences, this would be in breach of CVPR ethical guidelines, and likely ACM as well.
Regarding impact or prestige, nothing will currently (2020) beat an accepted paper at CVPR, no matter the type of presentation, if you are working in computer vision or deep learning related to computer vision. Having a CVPR paper under your belt would be great for graduate admissions.
That being said, on the flipside this also means that getting your work accepted in CVPR is likely to be much more difficult than to get it accepted at a normal conference. If this is your first submission to any conference, and you are doing most of the work yourself, then I'd wager your chances of getting accepted are in the range of 10-15% (If you submit an average paper that is written reasonably well, of course if your work is really outstanding, then disregard this rough estimate).