Do it, Emily. You won't be disappointed. I have 105 on my custom, because it was the only thing that works with the newer short reach brifters. I don't notice any difference between that and the Ultegra on my other bike. I have a 34 on this bike, as it's a compact and my go to bike for really big climbing rides.
Oh, and my bike with the triple? I've gone from a 25 to a 27 and now to a 28. Don't ask me about the mechanics, but DH did it all himself.