I've only used 2-piece. I like to "burp" my bag at the flange to get the air out. I think most one-pieces have filters to keep gas out of the bag (I personally didn't like using filters), or you can release air from the bottom opening. I can also put a fresh bag on the 2-piece without replacing the wafer itself if I feel like it.
OTOH, I understand with 1-pieces some people report fewer problems with leakage. Lots of pros and cons to both. The hospital will be putting you in their set-up initially, which will probably be a 2-piece. Later, you can start experimenting with different types of products (contact the Hollister, Convatec, Coloplast, etc. for free samples). There's also a facebook group called "The Real Ostomy Support Group" that's given me lots of good tips re. ostomy equipment and care that you can check out if you are interested.