Between the two interfaces, not really. Some people say that the Xonar Essence ST (PCI) sounds better than the Xonar Essence STX (PCIe), but if you ask me, that's just a myth. Some people claim the exact opposite, too...
Auzentech really has fallen as a company, but their sound cards haven't. They still put out some really amazing sound... it's just a little hard to get in contact with them if something does happen to go wrong. I've never had any problems at all with my X-Fi Forte, though... really well built card. I'd still HIGHLY recommend them.
Hmm... The Claros, you say? Honestly, I'm not a big fan of them. They may be decent sound cards, but I'm not a fan of HT Omega as a company, and there's one major flaw with that purchase: Auzentech has their X-Meridian 2G, which utterly stomps the Claro cards (IMHO... no offense to anyone... just an opinion), often for less money, too. The only thing stopping Auzentech, once again, is how small of a company they are and their lack of advertising.
The X-Fi can refer to two things - the X-Fi processors, or the X-Fi cards made by Creative. If you're talking about Creative, then yes - they have the same problem as HT Omega: slightly inferior cards at poor price points. A lot of people used to complain about their drivers, and although they're still not the best, they've come a long way and are at least usable now. Auzentech actually uses Creative's X-Fi processors (ONLY in their PCIe cards) on their own cards, and has vastly improved upon the drivers and performance in general.
As for a recommendation for your 5.1 setup, I'll have to recommend either the Auzentech X-Meridian 2G or X-Fi Forte, or the Asus Xonar DX. Both Auzentech cards should be around $110, and will basically do the same thing on two different interfaces. Actually, the X-Meridian is a full-fledget surround setup card for 7.1, while the X-Fi Forte is more for smaller setups, especially headphones, but supports everything up to 5.1 surround. Being a low profile card, though, it needs a breakout port on the back instead of each individual plug - look at a picture and you'll see what I mean. Really amazing card, though... both are unmatched for the price. If you don't want to spend $120 on a sound card, the next step down is the Asus Xonar DX or D1, depending on your choice of interface. Both are more of a general use card, and while they both put out some really nice sound, they're just not quite up to the same level as Auzentech's cards. Note, the DX needs a 4-pin floppy power connector... just so you don't miss that like I almost did. The next step down is the Xonar DS, I believe... just a lower quality version of the D1(X).