With more and more stories of people making music with their tablets (e.g. Gorillaz's "The Fall" album -> The entire album was recorded on group co-founder Damon Albarn's iPad), I'm wondering, can I safely make an electronic jazz album with vocals, with an iPad? Or should I still buy a PC/ Mac?
I would recommend it only if you care about portability, and only for recording (not for mixing, mastering, or editing). Damon Albarn did it that way because he was recording while on tour, and he needed something portable.
If you need to record while moving constantly, if you need portability, using any tablet (and interface) capable of running a DAW (you want to have all tracks time ordered) and recording at 24 bits and 44.1 kHz (at the very least) is a good option.
But that's only for recording. Mixing is another monster. Gorillaz's album was recorded on an iPad, but it was mixed in a studio using a more robust system. The processing power of iPads and tablets in general is still very limited, so there are big restrictions on how many tracks and effects you can run at the same time.
I'm wondering, can I safely make an electronic jazz album with vocals, with an iPad? Or should I still buy a PC/ Mac?
It's not safe at all to base all your production setup on a tablet. Unless your production is very simple (few tracks, few effects, few synths, few everything) you'll run out of resources very fast, and you will not be able to produce bigger stuff later if you feel like it. Even if you do your recording on the tablet, you still want to do everything else on a personal computer system.
I strongly recommend you to buy a PC/Mac. You can do some basic stuff in a tablet, but nothing close to what a PC/Mac is capable of. You can use the tablet as a complement to your PC/Mac production setup (maybe as a control surface, or to record stuff while you are not in your studio) but not as a replacement.
There's not big difference between a PC and a tablet. As long as an external sound card / sound recorder can be connected to it, it should be fine. Most tablets are fast enough as well for that since sound is much easier to pre-process than e.g. videos (for which I would recommend getting a PC with a good video card).
Just be sure that you get a USB sound card which is compatible with the device (there's plenty of them there).
The big difference will be the OS capabilities. If you use the garage band app on an iPad for tracking then you should be able to export the session files to a mac for mixing.
As others have said, you will need at least an interface accessory like iRig.