I have an old (I bought it second hand 7 or 8 years ago) sleeveless suit that only fits me at the lower end of my wieght range (ie definitely not at the moment). When I bought a new one this past summer I went long sleeve. The reading I did suggested that a properly fitting long sleeve suit is faster (which I took to mean in a suit that fits properly you get more advantage from the bouyancy than disadvantage from having the suit around your shoulders in terms of freedom of movement.

But actually the clincher for me is that I am a complete and absolute wuss when it comes to swimming in the open water if it is even slightly cold.