I’ll give you my thoughts as an owner of 26″ SS HT, and XC Full Sus along with 100mm 29″ HT:
I had a 26″ steel HT and bought a scandal 29er frame to try for trailquests and XC enduros. Built it up with kit off the 26″ HT plus 2nd hand wheels off here. Originally ran rigid carbon forks and the bike flew on road and smoother trails. Once onto rougher trails (Whinlatters reds for e.g.) the rigid fork held it back – great fun though and quick on the climbs.
Got hold of some FOX 29er forks (off here) and that’s how the bike has stayed for nearly a year. On road, forest fire road and smoother trails, it’s definitely faster. Riding with mates on a 100km+ event, they all commented that once the trail ‘smoothed out’ I was pulling away without really trying. The big wheels are a little harder to get up to speed, though the wheels I have are 1800g +. Lighter wheels would help. The big wheels roll over ‘bumps’ up to ~4″ better – think braking bumps and tennis ball size rocks. On more technical rocky stuff though, the wheels do seem to ‘catch’ a little and get deflected off line – doubtless bigger wheels aren’t as stiff. On that event I mentioned, we’d to ride Kirroughtree’s McMoab with minimum dabbing – I was rolling over small steps etc, but when it came to the tight turns, it was harder to get the bike round (XXL frame mind). The steep chute at the end felt great, cos going over the bars seems impossible.
If riding rocky Lakes trails I’ll use the 26″ wheeled bike. At trail centres, the big wheels feel so much fun – swoopy around berms etc, it encourages you to lean the bike over more (maybe have to to get round the bends?) and the cornering grip is better. On steep climbs, the front wheel doesn’t lift, but wheelies/manuals are harder. On steep descents, it feels far less likely that you’ll go over the bars. At 6’4″, this is probably the first mountain bike that I feel I’m sitting in, as oppose to perching on, if you know what I mean.
So, unless trails are tight, rocky and technical, the 29er is faster IMO. At Kielder 100, I was far from fit, so riders were passing me on the climbs, but then I would cruise past on flats, descents with little extra effort. Are they more fun on most trails? Depends how you like your fun – if fast, smoother and swoopy then yes, more fun. If tight nadgery and rocky – I’ll go with 26″ wheels.
Hope that helps and the pi$$ takers can flame away, ‘cos I’m off for a ride in the pouring rain – on my CX bike!