I know Travelocity lets you choose your seats.
I also know Expedia shows you how many seats are left on a flight, but I haven't booked a flight with them yet, so I'm not sure.
What I DO know is that Expedia tends to have more flight time options than Travelocity. I noticed this after I booked my Disney flight with Travelocity and went over to check Expedia and saw that THEY had the flight times I really wanted!
I book almost exclusively on Expedia now, but when there was a problem with my flights on Travelocity, the customer service was great!
The pricing isn't much different from both sites, so it's really a matter of preference.
HTH!