what do you think?

it depends more of the rear end, the longer top tube is mostly to give you more room, but for me manuals are better with a shorter one, i rode a 21 and now a 20.5 and with my actual frame manuals are easier

It depends on your height...a shorter toptube is generally easier to get into the manual position...it depends on your body style/height as to whether a longer/shorter tt is easier to maintain manual position.

I can only do manuals on my bike cause it's my bike :D

a top tubes not going to teach you how to do something, go ride man . you'll figure it out no matter what bike size you are on . promise.

its the rear triangle that makes a difference. longer= harder to start but easier to hold
shorter=easier to start but difficult to hold