I use pellets in the same way as Omi - mainly as a bribe and to check that they are eating normally. Leftover pellets rings alarm bells when they only get a small handful a day. Hay is much more important.
It's such a small part of the diet that I wouldn't be overly worried about which you choose, as long as it's one of the more sensible ones (vs full of junk & sugars / molasses ones). I use mainly Science Selective mixed with some Burgess, just so they don't get used to a single brand / recipe which then changes or isn't available when you need it.
Alfalfa is useful if you need to get / keep weight on a rabbit - so the young and elderly.
Indoor rabbits may also benefit from a specialist indoor rabbit pellet to make up for the vitamin D that they don't get by not being outside.