The Altitude is an excellent bike, that totally revamped my enthusiasm for hardtails. I've had mine about 18 months now, it handles so sweetly, and will really surprise you what it can do. And the 10 has a pretty decent fork and kit for the money when you take into account the frame is well worthy of future upgrades.
The io id, though I've only ridden one up and down the street, I have owned an Alfine hub for about 6 months and ridden it through all sorts. I can't recommend it highly enough! I'm not as keen on the io frame as I am the Altitude, but it certainly seems a quality bit of kit.
You won't go wrong with either at all, you just have to decide whether you want a bike with more modern geometry but conventional derailleur gears, or a more traditional geometry frame but with the bonus of the Shimano Alfine hub gear…
FWIW, the only time I've found myself lacking in gears on the Alfine was on a VERY steep welsh mountainside, when I could have done with one lower cog. I run 34/22 on my 29er (about the equivalent of 34/20, or possibly 32/18 on a 26" bike) and find the gearing spot on for just about everything. Yes, you will spin out on the road going downhill if you've got quick tyres on, but so what… I ride mountain bikes to take them offroad!
Oh, and at 5ft 11 (same height as me incidentally) you could go either 17.5 or 19 on the size happily I'd say. I'm slightly longer in the leg, and shorter in the body than average, and went for the 17.5 Altitude and don't regret it. There's only 15mm in the top tube between both though, so it's pretty close.
