First things first - the music comes first. Whatever format you're offering, if I can play it, I'll buy it. Whether it's a bullshitly ornate sculpture with a tape stuck up its arse or a cdr decorated with a "sharpie", if your music interests me I'll want it.
Favourite format - online. MP3, FLAC or wave. Cheapest, easiest, most convenient, least hassle, best. No argument. This is how I like to put my own music out for those and other reasons, and I'm more likely to listen to yours as well if it's online.
Second favourite - cassette. I've always loved the convenience, warmth of sound, underdog status and universality of cassette. Still the only physical format you can re-use if you want to. Personal preference - anonymous cassettes of anonymous Noise left on trains, in laundromats, doctors' waiting rooms, etc. Many people these days don't use tapes (some don't even know how, I suspect) so there's a nice "fuck you normal people" aspect to it as well. The thought of some unsuspecting soul picking up a tape, actually listening to it and getting something from it is wonderful.
Third favourite - cdrs. Cheap, easy, convenient, little hassle. No argument. Pro pressed cdrs are good but I prefer to burn them at home. If there is an issue with longevity, just rip the damn music.
Fourth favourite - cds. Purchase only - sadly, in Australia one has to make over five hundred copies of an album to get cds manufactured as opposed to pro-pressed cdrs. Haven't found anywhere in this country that'll give you less than five hundred cds. But if you're putting out your music on cd, brilliant. Long lasting, great sound, convenient to post and to store. Not a great fan of jewel case for what that's worth, I prefer something smaller.
Least favourite - vynal. Costly to make, costly to post, costly to buy. Pretentious vanity items, really. Not that that'll stop me buying them, but I'd better want the music first.