If you are comparing the 500 and 250 XCITING, I think I would prefer the 500 for extended 2-up riding. It will have the power to confidently move two riders (with luggage) in a speedy and confident manner.
I have the P250 and I am a "2-up" single rider and the 250 will get me to 80 MPH but will slow to 50 on the big steep mountain climbs to Tahoe. The difference between the 500 and 250 , in the Xciting, is mainly the engine difference. The 250 is lighter but has less power, the 500 is heavier with more power. The frame is the same, the wheels are the same. You can get ABS on the 500 and can't on the 250. Once you are moving, there is not weight difference.
I am all about confidence when I recommend a bike. Yes, the smaller one will get there but will it do it confidently? The 250cc Kymco's (all of them) are a great size for in-town use and some touring. The 500 is better at the freeway miles and for two-up. I don't think the 250 is any more or less nimble in-town when being powered by the engine. When you are moving the bike without power, the 350 will be easier.
