We own a BV500 and a GTS250ie. That is as close to the right comparison as you are gonna get prior to actually getting your hands on a Super 300. Assuming for a moment that the BV500 and the 300 Super have similar performance...the following should hold true when deciding on one or the other...
BV500. It is much better at high speed stability. I ride mine at 70-75 all the time. I have as much comfort at 70 on the BV as I do on the GTS at 50-55. It is much better at 2 up riding. It can also handle baggage cases better. It is noisier, rougher riding (stiff suspension), gets poorer fuel economy (about 60 MPG) and clearly not as good looking as the Vespa. Get the mid height Piaggio windscreen and Piaggio side cases (Givi for the top case) and make sure the fuel hose clamp problem has been taken care of.
GTS250ie. Very smooth, quiet, fuel efficient (>70 MPG). If you get a genuine Vespa trunk (we did), it looks good even with a baggage case. More nimble around town. Has a slight steering wiggle at 30MPH (all GTS's have this) and feels more like a skateboard on the freeway than a motorcycle. Can't beat the look though.
Hope that helps...