Mentalepsy: I never owned these platforms, but I've been curious about them for some time. Is this package worth owning?
Well, the emulators themselves come with no nasty DRM, except for the serial key you need to type in to unlock the full version. Technically, the emulators are free, but for Amiga you buy the licensed roms necessary for the OS to boot. For C64 you buy convenience.
You don't get many notable games with the packages, but the launchers are designed to easily let you add the floppy and tape images you might "come across" online. I think, they even recognize the more common games and set the emulation up for them. The whole "how to get games" is a fun area, as Cloanto links to some online database, which may link to other databases, which link to files hosted not by them, but by another party.
As for possible DOS vs. Amiga differences, as long as you stay away from lazy PC to Amiga ports, you might get something like this
Phantasie III DOS Phantasie III Amiga Better looking stuff with a mouse-driven interface.
Even C64 often one-ups the PCs of that age.