I know the trend is for fatter tyres, but for trial riding probably be better to have a 26" tyre between current 'normal' and full on fatbike. Something that weighed as close to a kilo as possible max and could be used with standard 135mm rear end and able to use an IGH, SS or keeping 5/6 cogs of a 9 speed cassette. So maybe a 3" tyre.

I also have a 26" uni, so very direct comparison of the merits of 20" and 26" wheel sizes (granted neither has suspension, which appears it may be very important for the smaller wheels) - arguably more direct than riding a bike. I'd tried off-roading with the 20 before I got my 26, and was thinking that muni was really pretty hard - even smallish bumps were causing me problems. Was amazed at the difference the bigger wheel made - far more than I was expecting. It really does roll over bumps far better.

However to get back to the 29er debate, my understanding is that the difference in rolling is far less pronounced - it is after all only a 10% difference from 26 (bearing in mind it's really a 28.5er) compared to the 30% difference between 20 and 26. You're also at the point where the disadvantages of larger wheels start to some into play. So as mentioned above, what's best depends on what you're doing. I have to admit I've actually become convinced of the merits of a 29er for a lot of the riding I do, without even owning one!

I'm sure at some point I'll get a 29er uni at any rate (the chances of getting a 29er MTB are rather low due to my investment in 26, and decreased disposable income nowadays). Unis do make you think about wheelsize a whole lot more, as there's a whole other level of compromise going on (bigger wheel means a higher gear which could be either an advantage or a disadvantage - bigger wheel also means more stable, less manoeuvrable and higher off the ground, so harder to get on and further to fall!)

Oh, and wow at epi's little wheeled bikes. Just wow!

I've seen some really wide small tyres.like a 20" pugsley,they'd be interesting.
and you can get decent susp forks for 20",from german:a.

There are swings and roundabouts. I don't recall feeling particularly disadvantaged while riding which is why I wonder how a purpose designed bike would go - maybe a project for summer?

A stopwatch may have revealed otherwise, but I was just out riding. The small wheels feel trickier in rocky stuff, but they do allow you to dodge a lot.

http://farm5.staticflickr.com/4148/4997654217_b699e31ca8_b.jpg

http://farm5.staticflickr.com/4100/4734692941_5d839e3077_b.jpg

Not to mention bashing my 45yr old Moulton round the 'Puffer course - sorry no pic of that.

(That orange Dahon has taken me over most of the Great Glen way - bit sketchy in places with the skinny tyres though.)

I do prefer proper large wheels for most riding though (and I do mean large :) ).

Looking at the wheels in isolation, obviously a small wheel will drop into holes etc, but if you use a longer wheelbase then the CoG isn't displaced as much so it won't feel too bad. This is nicely demonstrated by the original Moulton which has around 43" of wheelbase.