I just sold a bike with Ultegra and my current bike has Rival. The important thing is both flat out work. There are some differences in feel (Ultegra was really smooth on the sfifts, Rival is a bit more abrupt) but they're not a big deal. I haven't ridden a current 105 bike, but know plenty of people who race on 105 equipped bikes, do double centuries on it, ... all with no real complaints.

I think a lot will come down to personal preference. Do you like the shape of the hoods? SRAM's shape is different than Shimano's. Do you like the way each shifts (SRAM's shifting is different,and kind of counterintuitive at first). Do you want a triple? SRAM doesn't make those.

I wouldn't hesitate to buy a 105 equipped bike, personally, if it meant saving a few bucks.

JB