Personally, I’ve never found actual speed to be that meaningful.
I have ridden with a speedo/distance measurer on both my 24x3 and the 29-er, but never found it that useful.
Basically, a 36-er (which I don’t own) is going to be fastest, probably the only ungeared uni on which you’re going to be occasionally hitting bike speeds.
A 29-er with 125s will be OK for riding with bikes if they’re not focused on going particulalrly fast. It’ll be the next fastest practical unicycle to a 36-er, but will have the advantages of being compact enough to carry into buildings etc.
A 29-er with 150’s will, technically be slower, but, on rides involving steep hills, rough terrain or lots of traffic lights, can actually work out faster on shorter trips, due to the extra control of the longer cranks.
(That’s another factor with 36-ers- if you can’t idle the thing, or have problems free mounting, than any ride involving frequent stops at lights or dismounts/mounts, could well take longer than on a smaller wheel that you can control easily and freemount at will. My feelings are that it’s long distance riding on long empty roads where the 36-er can excell in terms of speed- that does not necessarily hold on shorter town journeys).
A 24x3 with 150 cranks, which is the unicycle I ride far more than any other, is clearly not going to go as fast as a 36-er or 29-er, but, for trips into town of around an hour, i find it absolutely ideal.
Unlike the 29-er, i can happily, after a lay of of several weeks, jump on the thing and feel totally in control.
If i want to push the speed to maximum and really spin, I’m a lot happier doing it on the 24x3 than on a bigger wheel, cos of the extra control, and, in the event of a UPD at max speed, the lesser consequences.
I like the fact that it’s easy to carry into buildings so I don’t need to lug a bike lock around.
I’m not suggesting that the OP gets a 24x3, just addressing the common misconception that for doing a bit of distance or commuting, that the only options are 36-ers and 29-ers: the humble 24x3 does have it’s own little advantages, and I’ve spent many happy years riding mine outside of it’s intended muni purpose.