I am going to buy a a new rod or two shortly. I want to buy a 2 piece for transportation but I know two pieces aren't quite as good. Would I notice a significant difference in sensitivity, casting distance or anything else like that compared to a one piece? Is a one piece particularly better?

I recently got my first one piece now that I have a truck, and not just my crappy little car to transport my gear. I do like it better, but I don't think it's a huge difference.

A quality two piece rod is just as good as a one piece of the same quality. Modern integrated ferrules are very good. St Croix for one makes a good travel series.

1 piece 100%, unless transportation is making it so you CANNOT carry 1 piece rods... 

A two piece rod always comes tip heavier than it's one piece counter part.  It's obviously because of the added weight in the connector or joint.

My favorite rod is a 2pc 8' Avid dont think one would notice if werent told. If 2pc fits your needs find a quality rod and go fish.

Depending on the surroundings you fish (trees and stuff on shore) you could get a longer 2pc rod that would give you more distance. Say you only have storage room for a 6' one pc rod or room for a 7' to 7'6" 2pc rod the longer rod throw farther.

I have two two piece combos, one spinning and one casting for when I can't bring the one piece rods. To be honest I can't tell the difference, maybe its just all in my head though. I have a lot of friends that say one piece rods are a lot more sensitive, but I feel they are about the same. Just go with whatever is more convenient for you.