Right, look at what's different then account for the extra cost there…
Frame, 853 Vs 520, possibly worth £100-150 extra.
Forks, Reba SL's with Maxle Vs Recon 351's, possibly worth another £100-150 again.
Drivetrain, slightly better on the 20 through upgraded rear mech and chainset, worth perhaps an extra £30-50.
Wheels, slightly better hubs and rims on the 20, worth perhaps an extra £30.
Brakes are the same on both bikes by the looks of it.
Bars stem and post, own brand stuff on both bikes anyway, but looks slightly lighter and higher quality on the 20, so worth an extra £20 maybe.
Tyres, Kevlar bead on the 20 Vs steel on the 10, but both crappy Mountain Kings, worth an extra £20 if you're feeling generous.
I make the value of the differences a little short of the £450 asking price difference if I'm being honest, though whether I'm undervaluing the 853 frame and the Maxle fork I don't know. I've not ridden a 520 version of the Altitude, but I highly rate the 853 version as I've had one for some time now. I suspect that in all fairness, the 520 version will ride very similarly, just with an extra 1/2 to 3/4 of a lb extra weight in the frame.
If you're bothered about overall value, I'd buy the 10 as for £1k you're going to have a blast on it. You'll certainly not get £450 more fun out of the 20 I don't think. BUT… If it's a bike you plan to upgrade over time, then the 20 would be the better bet, as with the 853 frame and the better fork, it's more worthy of a componentry upgrade in the future.
Have fun whatever you do!
