Hi! I definitely understand the trouble of picking between prereqs. I was in the same boat as you last fall. I took Chem 351 (the more chem version of biochem) and Physics 250 together last semester (Winter 2022). I definitely understand the want to pick between the two instead of taking them together but while manageable, it was a tough semester. 
Ultimately, the two courses differ in studying techniques and rigor. I’m sure it’s the same for MCDB 310, but Chem 351 was A LOT of memorizing while Physics 250 was more conceptual and computational. In MCDB 310 you have to keep up with a lot of material (almost 100 slides per lecture of information) but I think you ultimately get to use a note card/sheet for the exams (unlike 351 where you have to memorize everything) which makes it more doable! In Physics 250, you get to use a whole sheet of paper for every exam and can use all of those notes on the final (so 4 full sheets, front and back) which is also insanely helpful! 
Personally, biochem was insanely more work and effort while physics was just practice problems before exams. Don’t get me wrong though, the content for Physics is sometimes hard to grasp as well. Both of MCDB 310 and Physics 250 exams are all multiple choice. I feel like MCDB 310 exams are fairer than Physics 250, but both can throw curveballs at you, so it’s a hit or miss sometimes. If you are looking to ease into the year, I would lean towards Physics 250 instead of MCDB 310 because MCDB 310 picks up relatively quickly and it’s very easy to get behind without a good work ethic. If you want to get biochem out of the way and are willing to buckle down for the semester, I would pick MCDB 310. Since biochem is more memorization-based and physics is more quantitative/conceptual-based, taking them together is actually common for pre-meds to do if you are also considering doing that. But, then again, that would really require a good work ethic and time management. 
Overall, you know what works best for you and your pre-med journey. Everyone is different and I would pick MCDB and/or Physics 250 based on how much you think you can handle for the semester and how it flows with your pre-med journey.
Hope this helps!  Feel free to follow up on this Q&A post or make another post if you have any further questions!
  

Best, 

Mary, PMH Advisor