The SB Touch is designed for music and gives quality sound output, playing files natively up to 24/96 and for higher res it downsamples the files (halves the bit rate: 176 to 88; 192 to 96). It also plays just about every audio format known to man.
It isn't a portable device - you attach it to your sound system and either you sit near it to use the touch screen, or you use the included remote, or you use a laptop, iPhone/iPodTouch, or iPad to control it. You can get an SB Touch for $250 on ebay. Attached by it's digital out to a DAC it can give true audiophile level performance (way better than most PC soundcards, even the "good" ones), but understand it works better (more smoothly, faster) when using an external PC/MAC for the server software (you could attach your external drive there) and not it's internal server software.
The iPad isn't optimized for playing music, and I don't think you can get true hi-res out of it (i.e., not downsampled to "standard" res). It has the huge advantage of being portable, although in the setup you seem to be describing you would sort of be cancelling the portability factor. And it does other things not related to music.
So the products aren't really in the same category. Decide which feature set meets your needs. If you have already have a PC/MAC you can use as a server and output to your DAC, the iPad would be a killer interface; on the other hand you would probably get better sound and true hi-res from the SB Touch.