The GH3 will shoot slow motion as slow as 40%. My question is; would it be better to shoot at 60p and apply slow motion in post with Adobe Time Warp or Twixtor?

You should use that only if you need slower slow motion than what 60p gives you (i.e. If you wanted to emulate 120fps).

I slow 50P down by 50% in post and get smooth motion, so I guess slowing down 60P 60% should result in somewhat the same result. As I see it no twixtor or timewarp should be needed.

In P.P. CS6 I typically modify/interpret 60p footage as 23.976 fps which is usually slow enough for most purposes. I've used time warp also when i want to slow it down more although that can add artifacts.