Depends on the water temp, and your cold tolerance.  Me, I'm a wimp, so I like having a full suit in 60-degree water.  The negative, of course, is wearing full sleeves in 70+ degree water can seem hot.  For the record, full suits are a bit faster 
(4-5 min over IM distance
). 
 
My solution was to buy an item called "Tri Sleeves" from ProMotion 
(http://www.wetsuit.com/store/detail.cfm?product_id=401
), which is a jacket-like top that fits under a sleeveless wetsuit to convert it to full.  They're just $66 for the '03 model, so you could spend $130 for a sleeveless, and then $66 for the sleeves to have it both ways for under $200. 
 
Or, go sleeveless for now, and realize that you can add the sleeves later. 
 
I have worn my suit
(s
) in 2 races: 
2003 Black Diamond half IM, 60-degree water, I wore the sleeves to make a full.  Very comfy.  Maybe a third of the racers had sleeveless, and they seemed to be fine.  
(2 crazy guys went bare, but that's CRAZY
). 
 
2003 Great Floridian full IM, 77-degree water, I went sleeveless, and that was comfy.  Maybe a quarter of racers went with full, and I did hear of a few who thought that that was too hot.  A decent number went bare, which is more understandable, but I'd rather take the help when I can get it. 
 
I have two races coming up this year, both in 60-65 degree water, so I'll be very happy to have the full. 
 
Good luck!