The Cobra seems to get the best reviews. I just put a brand new 121" Cobra on my Apex, but haven't had a chance to ride it yet (obviously). I wasn't aware that they made it in a 1.5" lug though, mine is a 1.325" height. I don't have any experience with Nytro's, but I would be really surprised if it couldn't handle a 1.325" lug height. As for the gearing, I would suggest leaving it stock and seeing how you like it first. Changing gearing is a relatively quick and easy job to do if you are not happy down the road.
You are going to need a bottle of chain case oil and if your sled has a decent amount of miles (like over 7000 or so) it would be good to consider replacing the track driveshaft bearings and seals. If you don't have high mileage, it would still be a good idea to at least change the bearing on the clutch side... typically the chain case bearings last much longer because they are constantly bathed in oil.
I stretched my Viper out to 136" last year using TracksUSA rail and tunnel extensions... the rail extensions were great but the tunnel extension needed a little modification to work with the rear heat exchanger I added.
Jeremy