I've got a PODxt live, and it works fine for recording straight into the USB on my PC, as you say - they're very popular and very easy to use. I think the decision for you rests on what software you plan to use to record. I use sonar and have midi capability so I can set up my own drum loops when I need to...

The advantage I can see in the m-audio black box is that it has drum loops built in, which the POD doesn't - that might be useful if you're starting out.... one less thing to worry about to get up and running.

I've never used any m-audio products, so can't say for sure how good they are - it gets good reviews though. Some people dislike the POD amp models, but with a bit of tweaking I think they are usable for home recording...

try these:

M-Audio Black Box reviews

Line 6 POD xt Reviews (Pro version - same functions though)