I'd keep the Corolla until it died (or was too expensive to keep on the road) and save up for something better than an xA. Scions are geared towards teens and people looking for basic transportation; nothing wrong with that but you already have a Corolla.
I'd hardly call the tC a car for someone who wants basic transportation. It comes standard with keyelss entry, 17" alloy wheels, ABS, a panoramic sunroof, power windows, locks, mirrors. It also has steering wheel audio controls, a premium stereo, cruise, tilt, AC and automatic lights.
The Xa and Xb are kind of basic transportation, but even they come with more standard features than the Corolla.
Scions may be geared towards teens and twenty somethings, but I've yet to see a Scion piloted by anyone less than 35, most of the Scions I see on the road are being driven by 50 year olds. Toyota really missed the youth market with those cars.
I'm a twenty something, and I wouldn't drive a Scion unless you gave it to me. They aren't any cooler than a Corolla and quite honestly, there lack of a dealer network isn't something I'm wild about. While I could get a Scion worked on at my local Toyota dealer, I like to buy my cars and get them worked on at the same dealer and since my local Toyota dealer doesn't sell Scions, that would not be able to happen.