Temporary pump gains add fluid to the penis, which for me sems to pool under the skin, puffing out from the hard tissue underneath. I can still get and maintain an erection, but the extra size acts as an insulating pillow, cutting down on the good feeling for me. Of course, getting stretched by a fatter cock may provide additional stimulation for your partner, so you have to decide what is more important for this particular encounter.
And of course, remember that even bad sex is pretty good, especially when compared to the alternatives!
I do truly believe that bigger is better!