if weight isnt an issue then stick with the steel. They are more durable and less 'flexy' than aluminum ones. Weight isn't an issue for me either - at 240 before I put on my heavy boots, gaffs, gaitors, helmet, camelback, small sledge, harness, wedges, saw etc the extra few grams isnt really going to bother me. The nice thing about being a bigger guy is that you don't have too much trouble getting a good set on your gaffs - just get the right angle and you dont even really need a kick, just step on them.
Guys who have the lighter carbon spikes always seem to rave about them. I think a lot of these guys are lighter guys to start off with though. To me, the thing that makes for a comfortable long day in spikes is great shoes - with heavy steel sole plates, or you can weld an extra metal plate onto your spikes, plus great pads.
Shaun