Well... I don't have the sit & climb, but I do have the 2006 Lone Wolf Alpha hang on. What a stand. I just got through going through this decision process myself though on the sit & climb or a Summit Razor (almost identical to the Viper).
In the end, the Summit won out b/c of price and the fact that next years sit & climb will come with a larger seat frame. I'm 6'-1", 240 lbs and I don't want to feel squeezed into a stand, and wanted the larger frame, but didn't want to sink the $$$ into it, knowing it's not EXACTLY what I wanted. The Razor has plenty of room for me and I'm very satisfied with the purchase.
I did like the idea of a flat packing, lighter weight climber though. I also think the Lone Wolf cables have ALOT more adjustability in them than the Summits.
You should get plenty of opinions on this though....
-ZA