I would recommend selling 505 get a used padkontrol and buy a microsampler but look for the new lower priced(350$)...
However MS has some serious flaws such us you can't sample&loop immediately (like kp3 does) and when you choose a sample for keyboard mode you have to use the dial knob(very sensitive¬ precise...).
Keep in mind that in keyboard mode the pitching isn't hi-quality and cannot be compared with software or other more expensive hardware...
I personally bought it only for one reason :
the sampling modes are unique...!
PS.I had 505(and 606) and the only reason I sold it was the slow workflow...
PS2. If you always use a laptop I'll make I guide soon how to use the keyboard and the mousepad for controlling some of the ms parameters(midi related stuff)
PS3.can't wait for the 3.56-3.60 hack