Zen does not have a 5 band EQ... it has a 4. Which isn't bad.. you just set one of the missing frequencies to your main volume. Unless of course they crapped out on that as well... But in terms of sound reproduction, Zen Xtra does take the cake for best quality. iHP and Karma are also prefered for their "personal signatures"... and depending on what kind of person you are, the Xtra may not be for you. Sweet Spot does propose a great idea for you, which is to try out all these players before buying them. I second that as well.
All that lies below is just stuff on each individual player, from information that I've gleaned from others. I do recommend that you try these players out yourself, but if that isn't possible... A little extra opinion on it isn't bad either... But if you are already planning on trying these players out yourself, then don't bother to read all of this stuff..
Zen Xtra is a great player... mind you, it's a monster of size, but quality and durability are there (not to mention the LCD... I wish I had an LCD that big)
Removable battery... you can add on this... $70 remote.. or something... and record voice and play FM. Xtra does pack EAX features with it, and it's about the only one I've read to have included it (or maybe I've just overlooked a bunch of stuff... which I probably have, so just keep looking)
Karma is shorter than these players, but it is a great deal thicker. Pockets tend to be flat (to the curve of your leg) than they tend to be spacious... so unless you got hello baggio pants, the Karma won't fit as well. Not a big issue, but I can say I've never seen a girl with a Karma in her pants.
Karma has hella issues.. with reliability, so you'd best get some extra, extra insurance for this one if you plan to use it. Some people have claimed to go through several of these players while others have dropped them from shoulder height and had nothing more than a traumatizing moment of anticipation.
Rio does have this issue... with the quote "VaporWare" of Karma 40 gig, and Nitrus 4 gig. Announced a long time ago, these have yet to come out. THese new players are reckoned to be much better than the predeccessors (or at least the 40 gig, which is deemed to fix a ton of the bugs with the 20 gig), but they just never get released... Very bad because everyone is waiting for the stuff to come out, and not buying their products. And if you do buy them, you'll probably get screwed over when they do release the 40 gig. Oh well... I've been trying a bit to get Riovolution forums to become a bit like iRiver forums. They got at least 2 engineers that work for the actual company who answer questions, offer advice, and are curious about what their customers want. They havn't done too much to release updates, but they're getting there. Which is a very good thing for your player.
iRiver... beats the crap out of everything with the features (if you take iPod's wannabe cell phone features out). But, it's not really as if all those features will really all be used often at all... Great player... check out iRiver forums. There's a nice thread that has a link to this one page... it's in Japanese, but it lists all the new models set to come out in English. They got at least 12 new models coming out (which is a good or bad thing)
iRiver also is well known for it's great customer serivce... firmware specifically. Got an issue? Want a feature? If you find enough people on the forums who also need what you want, then they'll make a new firmware update to see if they can satisfy you guys. iRiver... unfortunatly... is also hella expensive... so unless you need all that great stuff, (and I mean NEED) iRiver might not be the one for you.