I've always had problems with figuring out the rfss smaller than 520, since they all weigh and cost about the same, but the 520 has way more power. I don't know if the 450 has better rideability on the EXC side due to some other differences, but I'd go for the 520 just for the fun.
Even the SX version is not some fire-breathing monster if you want to chug it (although the 4-speed 'box (and, hence, tall 1st gear) on the SX does make slow-speed maneuvering a bit tricky).