is this spooky or what. I too am 39, 5’7" and learned on a 20 before moving on to something bigger. Are you in fact me from an alternate reality? ;0)
This year, I decided to move on from my trusty 20 inch which had done me proud for 15 years. I bought a 26 inch MUni, then a new 20 inch Nimbus, then a 24 inch Nimbus, then a Coker. So that might qualify me to answer your question, but not to be a financial advisor! :0)
Wheel size. Bigger wheel = more speed per rpm. BUT as the sizes get bigger, an extra couple of inches makes less difference:
For a 4 inch difference:
A 24 inch is 20% ‘faster’ than a 20 but
a 28 inch is only 16.6% ‘faster’ than a 24.
For an 8 inch difference:
A 28 is 40% ‘faster’ than a 20, but
A 36 (Coker) is only 28% ‘faster’ than a 28.
Bigger wheels roll better, but are harder to steer.
Bigger wheels need longer cranks to achieve the same ‘gearing’ for hills.
If I had only 1 unicycle, it would either be a 24 (the ideal compromise between the manoeuvreability and idleability of a 20 and the ‘speed’ and rolling ability of a 26) OR a Coker (because nothing else is quite as… well nothing else is a Coker.)
UK prices, a pair of cranks = £10. US = a few dollars. Don’t feel obliged to decide now. A 24 inch wheel with 150mm cranks will do all you want it to, and you can always wallop some shorter cranks on for fun now and again. I change my cranks from time to time just to change the feel of the uni.
It is unlikely that your next uni will be your last. For maximum versatility, I’d get a 24 inch Nimbus/Yuni or similar, with 150 mm cranks, put some half-decent platform pedals on it, and ride it lots. Then I’d know if my NEXT uni should be bigger, faster, fatter-tyred or whatever.
ON MY 24 with 150 mm cranks, I climbed about a mile of track at a posted gradient of 1:7. and I have ridden some pretty difficult forest tracks and the like. On the same uni with 110s, I happily ride rough but level terrain with occasional short steep hills, and 5 mile rides are easily achievable.