US tracks 'usually' have much larger jumps. One of the main complaints leveled at Xray buggies and truggies, is that the shocks could use a bigger bore. Euro tracks are usually more technical and involve less airtime, negating this need. That's all. If your track is not akin to a motoX track, you have nothing to worry about. Check out the euro 2007 finals for an example of a great euro track with almost no air, and requiring a very demanding technique.