Hi there,
I also was accepted to both programs and decided I'm going to attend LMU (pre dental). I would recommend asking to speak with a student currently in the post bac program. That's what really helped me make my decision. The reasons I went with LMU were basically:
1) The shorter time frame, which is good both for getting your prereqs done faster & the 3 courses helps show you can handle a heavy course load. Speaking with a current student it sounds manageable and like everyone in the post bac class was doing pretty well in the program. I believe many in his class even took a 4th class both semesters. Additionally, the student I spoke to still did well (4.0) while working a 5-10 hr / week job. I too am nervous about 3 science classes / semester since I didn't do any in undergrad, but if you look at it as a full time job (+ probably 5 or so hrs / week working/volunteering) and work hard, I'm sure it's doable.
2) Smaller class sizes. Speaking with the reps at USC, intro science lectures can bee 200-300 students, whereas at LMU the maximum lecture size I heard of was around 50-60, and labs are 10-15. That sets you up well for getting to know the professors for recommendations, research (if you stick around after year 1), etc. I've been out of school for 3 years so the small class size/interaction with professors that you get at LMU was a really big draw for me, as opposed to being thrown into giant lectures at USC with tons of undergrads who are all trying to beat the curve.
3) Cost. USC is twice the price, plus an extra year of living expenses. I'm trying to minimize the loans I take out before going into dental school.
4) Location. I live in the West Side so LMU is a lot closer. This wasn't a big factor but it's a nice perk.
After considering all that it wasn't really a tough decision. In terms of acceptance rate to med/dental school, the LMU program is pretty new, so perhaps that's why they don't have a ton of stats on acceptance. USC's program has obviously been around longer + may have some name recognition, I'm not sure. I went to a well known university for undergrad so I'm not that concerned with prestige in my post bac classes. I just wanted to do the cheapest 4 year school I could find, and the state schools in the area didn't really work for me. In the end I think it's less important where you take the classes as what kind of grades/test scores you get, so if you work hard at either program I'm sure you'll be fine.
Good luck in deciding!