A very good question. Assuming both are autos, the Lexus IS250 is smoother and more reliable (it's actually one of the most reliable cars you could buy) but you have to watch what wheels and tyres it comes on. It's far better on 16s or 17s than on 18-inch wheels. The Jag will be a bit more exciting and a bit faster. Wheel and tyre choice less critical, but still softer on 17s or 18s than on 19s or 20s.