I've used both quite a bit, as far as prices and customer service they are pretty well matched. Sometimes there is specific brand or an oddball -an adapter I'm looking for and one will have it but not the other.
One deciding factor for me is that Jegs uses Fed-Ex and and Summit uses UPS.
I know all areas have different experiences with the two shippers, but at my house FED-EX ***** HUGE DONKEY BALLS, they are contractors and there is ZERO consistency as to which of the 15 or so drivers I've seen come here or how many days it takes, what time of the day they arrive, or if the arrive at all.. I've had to battle with them a dozen times over the past 5 years, when drivers flat out lie and mark the tracking as 'customer not home' when I had made a point to wait all day in a wide open garage working on something I'm waiting for the parts for. I know I'm on the tail end of the route in the woods, and they get lazy to come so far and know a different driver will have to do it the following day. They also throw packages in every different nook and cranny all over my property, sometime in the trailer, under the rear deck furniture, beside the garage, on the hood of my work truck, all kinds of dumb ****, so when the tracker says delivered if it's a small package I get to easter egg hunt for 20 minutes to find it. All because my covered front porch is apparently too far from the driveway to walk to.
When they make these lies up, there is no door tag, and the package did not require a signature, so if they did infact show up, then there would be a tag or they would leave the package.
UPS on the other hand 'for my route' is fantastic, the driver Greg shows up everyday between 3:30 and 4:30, puts packages in the same spot, BS's with me sometimes on whatever project I'm working on at the time, and is all around a great asset to me getting packages delivered and picked up with a high amount of reliability.
So in conclusion I always order from Summit these days unless only Jegs has the specific item I'm looking for, based on the single reason that Summit will ship via UPS.