I am 6 feet and 208lbs and also ride a Gary Fisher. It is a 19" (large) Piranha (genesis/gold series aluminum) and the geometry fits me perfect. Better than anything else I tried ( Trek, Specialized, Giant, Fuji, Cannondale, ect). I have long arms for my height so the longer top tube helps with that. A 21" (XL) genesis frame should fit you pretty good. I have never had any frame issues with mine as far as strength and durability goes, plus they have a lifetime warrantee anyway. I hear Fisher is real easy to deal with when it comes to warrantee issues, often times they will give you the next model up with a choice of color. The frames are made of internally/externally butted 6061 T6 aluminum and have well placed gussets and quality welds. I would recommend trying a 21" genesis frame out, the Cobia that you mentioned is a real nice bike. I don't know much about 29ers, but if anyone does it's Gary Fisher- he's been making them longer than anyone else. I think the combination of the "stretched out" geometry and the 29" wheels would probably ride real nice for a guy of your size. Good luck.
p.s. Get the Fisher, we need a few more in the club!!!