Good choice, i was going to reccomend a 29". Your options are:
a freestyle - a nimbus of some sort would be good
a muni - Qu-Ax downhill is on offer at £185 i think, very good for the money
distance - you could just about afford a 36 but you’d need atleast a better saddle before you could really use it, which puts it out of range, 29 is a better idea particularly if you’ve only ridden 20" before.
Something novelty - giraffe, two wheeler, BC wheel, ultimate wheel
Really depends what style of riding you want to do.
freestyle not my type.
the qu-ax muni is almost exactly the same as my qu-ax trials:( i no theres a dif but still my parents will moan. " the both the look the same to me!" and all that **** lol.
36 will be bad for stop starting on a paper round:(
novelty things only stay novelty for a lol while:(
I agree with your logic on all of those, the only other thing i can think of is a 26" muni which will be a bit futher removed from your trials uni and give you fairly decent speed for getting around. Again i would reccomend the nimbus muni. Pashley are a decent brand but they’ve failed to move with the times and their stuff is outdated.
Seems like you want a nice manuverable commuter uni. A 29er will be great for that, or a 26 with shorter cranks. But I would just go right to the 29er.
If you really want an amazing 29er, KH has this 29er for sale. It has a very light and strong everything on it. I new modified seat for a comfier ride. Splined hub and crankset for tons of strength, and the frame will never brake on you. =p
There is also the nimbus 29er. It has regular cotter-less hub and cranks, which are still great for commuting on. A good frame and seat, and has a ncie price.
Torker also has a new line of unicycles called AX. Not much is known about the Torker AX unis yet but from the pictures, they seems very well for commuting around on.
EDIT: If you do end up deciding on a 26inch uni for some reason, make sure that you get a decent road tire on it and not a sluggish knobby tire used for off-road. It will make the riding a lot easier and smoother.