I went with the YTZ7S... it's the same battery Allen has outfitted all his demo bikes with and many other people here have also moved to this battery. Over the stock battery in my 450 SX the 7 has MUCH more CCA and does much better in cold weather. I left my bike outside in -15 degree weather last month with the stock battery... didn't even get a single start out of it before the battery died. Went inside, swapped over to the 7, left it outside again in -20. Next morning she fired right up. I got 10 fires throughout the day before the battery started to show signs of dying (fire up, run 5 seconds, kill it... no charging the battery).
With all the mixed press on the Lithium batteries I'll take a nice dependable lead acid any day.