I haven't tried the regular HM5 pads, but im assuming they're the same size diameter as the thick ones. Mine were at first a little snug and i had a hard time putting them on. You definitely have to be patient. I tried (gently) stretching them out and it worked. Once i got them to fit, they stayed in place extremely well.
I have heard the HD439's briefly at a store (so take my words with a grain of salt) and in my opinion the Q40's sound better. However i did find the 439's to be a lot more comfortable and much better for portable use. It sounds slightly darker than the Q40's and doesn't quite have the same bass tightness and rumble (especially in the subbass). I find that the Q40's excels at electronic music (specifically electronic rock) and rock. Up to you really, but since you want it for home use I'd say get the Q40's.