I also like 21's in my SC8. BUT, of all the 21's I've ever witnessed die early deaths, pretty much all were VSPEC's.. They make good power, but I personally would be wary - especially of a used one.. If it were between a used VSPEC and the new stock engine, I might go with the stock 28 just to be safe..
Power-wise the stock engine will be good for drifting and bashing around. The VSPEC would have more top end.
Don't forget too - with the VSPEC you need a starter box. And turbo plugs, and preferably a matched pipe..