I used Priceline once and I wasn't happy about leaving Honolulu at 5:30 in the morning, especially when I know there are lots of flights leaving for the mainland in the late afternoon. Heck, a red-eye would have been better! I guess I shouldn't really complain because the price they gave me was significantly cheaper than other places. But that was before Hotwire came out.

I've used Hotwire since then, and though you don't know the carrier and times before you purchase, I wasn't really too worried because they tend to use major carriers anyway (but then again I'm in Hawaii where most smaller carriers don't fly here).

Basically if you're flexible and don't care what time you leave or arrive, then both are okay.