Will it be easier to maintain a high gpa at ucsd or ucsb for premeds?
All depends upon you. Both are going to be competitive for the Pre-Med requirements so pick the school where you will be most happy attending since happy students will be successful students.
but ucsd has a reputation of being more cuttthroat
All UC’s will have “weed out” classes even UC Merced, so academically neither campus will be more competitive than the other. I have heard “cutthroat” more in reference to UCLA/UCB and there are many Pre-Med students on both of these campuses that are thriving so you cannot make a blanket statement on a schools reputation unless you have first hand knowledge. I have also found that students that are not doing well or enjoying their college experience are the ones that tend to complain the most so the “squeaky wheel” syndrome seems to perpetuate the so-called reputation myths that many students always hear about.
Only you can decide for yourself since you are the one that will have to live with your decision. Both schools will get you the opportunities to do well, it is up to you to take advantage these opportunities.
I doubt you’re going to find someone who has completed 2 undergrad degree, one at each of these schools. So we’re left with data to guess. The SAT average at San Diego is 1350 and the 25th percentile is 1250. At UCSB the corresponding numbers are 1330 and 1210. So you can say the SD kids are slightly better students although the difference may fall within the margin of error of the test.
There is no doubt that between some pairs of schools you can say the competition will be stiffer at one than the other, but between your pair I don’t think anyone can tell with the publicly available data, other than that you have anecdotes.