You should try both XCP and NXT AE to see the difference in speed. XCP developers also had an issue with the Bitcoin developers after the latter changed something in the Bitcoin protocol which prevented XCP developers from doing some features or somehow limited their options, not sure about the details, but the point is, any DEX riding on top of Bitcoin network will always depend on good will of Bitcoin developers, and that doesn't inspire confidence in investors' minds. Oh, and did I mention that order execution is slow?
Bitcoin does only one function.
XCP does another function.
NXT does both of those + 2 more, and adding a few more in the nearest future.
the issue is long solved and was much smaller than expected.
I think the biggest advantage of counterparty protocol is that you do not need to go the way btc-> nxt -> asset. you can simply buy the asset with btc.
the "currency" (xcp) of the protocol has a completely different function. it can be taken in escrow on the blockchain (until now impossible with bitcoin). It will be probably in short term be used for betting, mid-term for CFDs and long term as insurance.
BTW I used NXT AE and I was quite impressed by the easy to use functionality as well as the speed. kudos.