I have done Steelhead the past 3 years so I've had two swims there. I only have a full suit and it is has worked out just fine for me. I just went back and looked at some of the pictures my wife took of the swim start for my wave and I would say that the ratio was 2:1 full sleeve to sleeveless in 2010 and 2:1 sleeveless to full in 2009, so it can easily go both ways. I'd look at the other races you are planning on doing and base your decision on that. I sort of wish I had gone with a sleeveless suit as a lot of my swims have been in inland lakes which tend to get a bit warmer than the big lake.
--------------------------
IMWI 2011 - check - 11:54
IMMT 2012 - check - 10:49
Michigan Titanium 2013 - 11:50
http://www.ironmanbythirty.com/