For me, summit's website is just better to navitate and the search results/engine are better. I like the old layout better before the new one, but eh. Also their customer service, in my experience, has always been awesome. I bet I've bought over or damn near $10,000 worth of crap from them over the past couple years, and as long as they keep doing what they're doing, they'll continue with my business. Jegs... I don't have a chevrolet, so I don't really care. Buick specific stuff/speed parts I order from TA, I just use summit for general stuff like engine paint, gasket kits, bearings, etc. Most expensive thing in recent memory I've purchased at one time, I think was my comp 268 cam kit that was in my blue motor two years ago. Oh, free shipping over 100 bucks is a cool thing too. Clear indication of when it ships, reviews are accurate, if you ask a question, it usually gets answered within 24 hours either by them or an informed shopper.
In all, Summit > Jegs.