Hitting flat is 'easier'. You can do it from more stances, and recover easier as well. Top-spin is a bit harder, less control and badly done will give the adversaries an easy ball, but it does give you the advantage of being able to get a slight more safe net clearance without being too high when the adversary will volley.
You can see this in most of the pros that they impart some top-spin (not tennis like though) so the ball drops more into the feet of the people at net.
(one thing I will say though, if you are playing against someone with a very 'good hand' at net, specially with dropshots, don't use top-spin. This makes it much much easier for them to do a drop shot than if flat.)
(ps: regarding the ball bouncing from the wall with top-spin, while true, I really think this is a bad advice since very very hardly when you are in defence will you play a shot that will pass both players, get close to the glass and rebound high. Actually, same speed ball, with and without topspin will probably get less rebound as the ball trajectory when hitting the back glass will probably be already descending while flat will be ascending).
