I am pretty sure that the differences are:
1.) Technical IRTA positions, according to the website are for training "future research support personnel," while Post-Bacc IRTA positions have the goal of training people who intend to continue on to graduate or professional school, (applying is actually a job requirement, and there's a lot of flexibility for traveling for interviews and such)
2.) Technical IRTA is a 2 year commitment with the option to renew for a third year. Post-Bacc is 1 year minimum commitment, with the option to stay for two (some labs will only be interested in you if you're coming for two years, this mostly depends on how much of that time they are going to have to spend training you.
I'm a Post-Bacc IRTA, and I would say that in terms of prepping for the MD/PhD track, that's the better path. That being said, there is a lot of variability in experiences here. I've stayed for (a little more than) 2 years, and will have multiple publications including at least one, and possibly two first authorships. Other people that I have talked to don't feel like they got much chance to initiate their own experiments or participate in design, and were more glorified (and badly paid) lab techs. If you're thinking about it, make sure you get to talk to the current IRTAs in the labs your applying to to see where on this spectrum their experience falls. If you have any other questions, feel free to PM me.