I have owned a Superfly 100 SL for a few months now and I was just able to take a Fuel EX 29 9.7 for a spin on home turf this evening. I hate to just go ahead and put the kibosh on all the future "which one is right for me" threads, but each of these bikes hits the nail on the head for the market that Trek is marketing them at.
My SL is a rocketship, lightweight, stays planted, climbs well -- it is just slack enough that I find the front end wanders a touch coming off of a 71 deg HA 29er but no problems with front end lift climbing. Very capable on the descents but the bike isn't overly "playful", it was designed to be fast and it is that, going up or down. A certain review called it out as being a wet noodle, which it is not -- but it isn't the most laterally stiff bike I have ever ridden for sure, but it isn't enough to be bothersome unless what you are really looking for is an AM/Enduro rig, which the Superfly just is not. It is an excellent and lighter weight compliment to the Specialized Epic, Trek has a great XC race/endurance bike on their hands here. I was a bit unnerved by the way it rode early on until I figured out how to quantify it, it's a bit like driving a supercar at 30mph down a twisty canyon road, that is to say.. boring, right up until you put the hammer down. The SL likes to be ridden fast, the harder and faster you hammer on it the more confidence it inspires and the more speed you realize you have on tap.
The new Fuel EX 29 on the other hand... well... funnily enough it is exactly what Trek is marketing it as, an AM/Trail rig. It will make a great all mountain / all day / epic adventure / light enduro / super-d bike. The suspension strikes a good enough balance that the bike could be raced XC/endurance and wouldn't give much up over a more hardcore race rig -- in fact it would make a great bike for a rough/chunky 50+ mile race. I have seen a few reviews of this bike say that it is very playful, "like the 26" version!" they say, don't be fooled, it does handle like a 29er, but it is definitely on the playful side. Especially with the shocks set to Descend mode, with your weight back a bit it becomes trivial to loft the front end and keep it up in the air. It descends with the stability that the long'ish wheelbase would suggest it has, but does not feel like a big bike when carving through a tight switchback. This bike is stiffer in the rear end than the Superfly, but they are not worlds apart -- like the SL the EX 29 also isn't one of those crazy laterally stiff bikes that will take a fast, rough, off camber corner and feel like it is on a rail, but any flex is not easily perceived and does not detract from the handling. The Fuel EX 29 would serve the one-bike-quiver role quite nicely.
Superfly 100 SL in a word: FAST.
Fuel EX 29 in a word: FUN.
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