Pretty much as above regarding 27.5 Vs 29 - I'm coming back to MTB's from a 15 odd year gap, I was on 26" wheels back then.
I've ridden a few bikes and have decided on 27.5" wheels. For me they are much more manoeuvrable and much more responsive in corners, in tight, twisty trails they turn easier, they're also a bit more playful. Lastly they are easier to get up to speed.
29ers roll better, will keep momentum much better than 27.5 wheels, and (if you have fitness to keep them up to speed) they'll be faster. However they turn slower and are harder to get back up to speed if you drop off the pace.
One thing I'll say, is that 27.5" wheels with 'plus' size tyres, i.e. 2.5-3.0 tyres is a brilliant combination - the radius is bigger making then roll more like 29ers and the grip levels are brilliant. The first thing I'll be doing on my new bike will be seeing if a set of 2.5 wide trail or 2.6 tyres will fit.
You need to ride a couple of bikes to see what you prefer, but neither is better - they're just slightly different
I'm 6ft 3 and prefer 27.5, there are short riders who prefer 29ers so height has no bearing.
As for shorts, loads of MTB sports come with removable liners, they're normally basic thin lycra shorts with the padded crotch/arse area. Just don't go full lycra unless you're doing an XC race