If you're looking at rolling objects, you ahve to also consider anguler kinetic energy in addition to linear motion. If you neglect friction and air resistance, a block will fall quicker because it only has linear kinetic energy.
Your theory is meant to be your own theory... and tbh you're better to be wrong so you can discuss it more. It's a lot easier to discuss why you were erroreous than it is for you say 'I won bitches'.
If you want to legitmise your theory iwth maths or something, just draw the free body diagram(s) relevant. If you're going to go the easy route and intentionally be wrong, then just leave the important diagram out

Though I don't think this prac is a good one, it can be done using just maths (Something like finding friction coeffecients has to be done experimentally... tan(theta of inclination) = mu(s) for example is an easy one. If you do MM it should be a simple calc/trig problem for the fastest ([TH] represents theta, or the angle of inclination of the plane):
ma=mgsin([TH])
a=gsin([TH])
You want maximum a, use calculus to find the derivative and set it to 0:
a' = 10cos([TH])
0 = 10cos([TH])
[TH] = 90deg.
So it's fastest on a flat drop, no inclined plane. Process of horizontal speed is much the same.