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.)
CRANK SIZE:
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.
OVERALL VERDICT:
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.