ps6175

So after much loitering and reading I've got my heart set on an aqua racer 500m as they sound like one of the closest to gen reps available - however what I can't seem to work out is the difference between the 250 ish reps and the 450 ish reps is this a movement thing ? I'm not one to post without researching first but I'm really at a loss here ( noob thing ) I was going to order from aoo but looks like they do not ship to uk Can anyone throw a bit of light on this for me as in which model to order and which td is happy to post to uk Great forum and thanks in advance