Hey mabajas76!
Although I haven't done UMEP myself, I know a few people that have and here is their piece of advice.
The first question you want to ask yourself is what do you think you are more likely to major in? (Physics or Maths?)
Here is just a summary of why a good chunk of my friends picked UMEP Physics over UMEP Maths:
The main reason all my friends picked to do UMEP Physics over UMEP Maths is that they wanted to enter the "Accelerated Mathematics" stream (MAST10008 and MAST10009. Note you need at least a raw of 38 in Specialist Maths). Now the big reason they did this over UMEP Maths is that by doing the AM stream, they would avoid a subject called "Real Analysis" (MAST20026). I have only been at Unimelb for one semester, but based on everyone I have talked to in regards to this subject 'Real Analysis', it is a killer. If you are thinking of taking a maths/mathematical physics major, my friends highly recommend holding off UMEP Maths and doing AM if you can in the first year just to avoid RA (Note: MAST10009 acts as an equivalent to MAST20026. You also save a subject slot can have another free spot to do whatever you want!).
If you do decide to do UMEP Physics and AM, you won't be able to do any Level 2 Physics subject except Lab and Comp 2 (as you would have not successfully completed all first-year maths subjects by the end of first-semester the first year). I also have a friend that did UMEP Physics last year and last semester he did Calc 2 and Linear Algebra Adv. This now allows him to take any level 2 maths/physics subject next semester as he meets all prerequisites. (Note: He is planning to do a physics major so he doesn't need to do RA)
Now on the other hand, if you do take UMEP Maths, you can absolutely take Level 2 maths at the start of your first year! This would be good for you if you are really keen on maths and really want to be extend your maths knowledge. The only problem is that you will definitely NOT be able to take any level 2 physics subjects as you would have not completed Physics 1 and Physics 2 (and you also cannot do them concurrently hence you have to wait for the second year to do level 2 physics subjects). You will also be in a position that you may have to take Real Analysis if you are intending to study a maths/mathematical physics major (which also means you will get a subject slot used). If you don't mind taking RA and have a strong passion for maths, I would say go for it!
However, if you want to "follow your passion", you should do UMEP Physics, as from what I heard it is a lot more interesting with hands on Lab sessions as well. But be aware that the workload is intense, and its way harder to get a 5.0
In regards to this, I sort of disagree with this being more intense and harder to get a 5.0. If there is something you should know, the Physics department here scales the exam quite nicely (I'm pretty sure they scaled my exam by at least 10% this year😂 in comparison to the maths department which barely scaled). The Physics exam here is only worth 60% in comparison to the Maths exams which usually are 80%. But I do agree with one thing and that is the is a larger workload (much more content) than what you would do in maths. It really is a matter of picking which one you are more comfortable with, more assignments and labs but less exam weighting or fewer assignments and high exam weighting. (And of course, what you are more passionate about). The labs can get a little intense because you do have to finish everything in 2.5 hours, but personally, I found them to be really fun! However, if labs don't fancy you maybe UMEP Maths might be better.
TL;DR
If you plan to do a maths/mathematical physics major -> UMEP Physics and get a high enough raw in Specialist Maths to do AM so you can avoid Real Analysis and have a free science elective spot to do whatever you want.
If you don't plan to do a maths/mathematical physics major but want to start doing level 2 physics asap (Don't do unless you have strong maths abilities) -> UMEP Physics and do Calc 2 and Linear Algebra in the first semester.
Any other option/you don't mind doing Real Analysis -> Do what you are most comfortable doing!! Pick the one you feel more enthusiastic about because you will ultimately do better in it.
If you have any questions on UMEP Physics, feel free to post them on this thread (my friends would be happy to answer any qs!)
Hope that sort of helps!