It really depends what control method you currently use.
If you're keyboard, then I'd say wheel first. A lot of the S2 cars can be very twitchy and hard to control with pure digital controls. Get some wheel practice in so that when you do get S2 you'll know what you're doing.
If you're mouse and can control the XRT well, then you could probably get away with S2 first. But again, some of the cars will be harder to control.
A lot of the fastest racers around are mouse users though, so it is possible to do well without a wheel. I can't do it, but then I pretty much suck with a wheel too