I'm not personally familiar with "Parallels" (or even iPads in general), but in my experience with "virtual" machines, they are almost always very slow and resource hungry. This is because they act as an emulator. This is fine when (for example) you want to emulate a C64 or Nintendo on a PC, but emulating a PC architecture on an iPad ... well, I'd have to try it and see (hey, I think I have an excuse to buy an iPad now
)
The difference with Crossover for Mac/Linux is that it is not an emulator, rather it implements the Win32 API almost directly on those platforms. With Emulators (like VirtualPC which I believe Microsoft bought out) you're essentially booting the target OS on a virtual machine; Crossover instead replicates the API directly, so it's more integrated into the actual Mac/Linux OS.
All of that said, an iPad is a touch-centric device (I believe that's a word) and though VDJ does well with touch-screens, it might not be the best in some situations (drag and drop, etc)... the audio on an iPad should be sufficient, and then you just have the issue of connecting a storage device (for your music) but that's another thing that has become much easier in recent years (USB To Go adapters)...
Funny I'm bashing the "Virtual" emulators, when in fact I named VDJ "Virtual DJ Studio" because in its own way it is just that - a Virtual "studio" for DJs... emulated on a PC. (Note it is not a "Virtual DJ" - that's another program. I can't write a program to do all of the things a DJ does (nor do I think the competition can). I can however write a program that does what the
Studio does FOR a DJ. Hence the mixer-board layout).
But it's not emulating one computer architecture on another, it's emulating the real world on a PC, which tends to be more efficient as that's kinda what computers are supposed to do - emulate the real world, making real world tasks easier
