If you like the look of the m series units then I would go ahead with one of those. Lots of people really like them. I only have an m702 not an m4xx series but what I would suggest if you're choosing between the two is to go with the m4xx because they now have PCCs for them and they will soon be coming out with cartomizers too fit them which means you'll have another option without having to
buy a whole new kit if you end up deciding you'd rather have a 2 piece than a 3 piece.
As for the 510 if you have no experience with any of these but would like to try one I know that
madvapes has a "disposable" 510 clone that is only 9.95 and is rechargeable. Seems like a good cheap way to see if you'd like that style or not. I haven't ever ordered from them but if you use the search function you'll find a thread here on the forums about them.