Thank you for asking. I'm not very "technical"; I'm more "girly". Specs on both bikes would be more specific. However, since you asked.....
The Honda, at 600 cc fuel injected, takes off like a bullet. It will cruise all day long at 70 mph on the interstate with no damage to the motor. I have heard that some of the smaller Hondas, like the Reflex, are freeway legal, but aren't made to withstand high speeds for long periods of time. The Honda can beat most street cruisers and a 1000 and 650 Vstrom at takeoff, but of course the larger engines will win in the end. I usually keep my takeoffs slow so as to not damage any strangers' egos. Being whipped by a scooter-driving Grandma might be too much for some people to take. The Honda is a smooth ride and is very comfortable with no vibration. My husband has had my Silverwing up to 100 mph and then he felt vibration so he slowed down. He has been riding motorcycles since he was 14 and raced motocross; I would not recommend that speed test for most people, including myself.
The Honda is very versatile; I take it in the mountains and with the ABS don't have much trouble but must concentrate on the downhills. It is fairly high-geared, so offers little slow-down power going down steep grades; I have to brake before the corners and sometimes engage the gearing around the corners rather than coasting. It doesn't corner like a regular bike due to the long rear end. Counter steering doesn't work quite as well on it as on a regular bike.
I am not sure of the difference in engines in the Honda and the MP3; I'd have to look at the specs. However, I know how they feel. The MP3 500 is a little slower at takeoff, but pulls me down the highway just fine. It has plenty of passing power. It has a big rear wheel, and when it catches its speed it almost feels like it needs those two front wheels to hold it down. I was down in South Texas last fall and was told by the MX guys that people would covet this scooter for its engine. It got big smiles from all riders. I had big locks for it. It's speedometer goes to 90 mph--I had it on the interstate once and wasn't concerned with not having enough power at all. The engine on the MP3 500 is different than that of the 250 and 400, but again, that would be something for the engine experts to discuss. I was told that the 500 can go down the highway all day and was developed for people that wanted a scooter more like a motorcycle. Look up the Gilera Fuoco website for very detailed information on this bike, because that is how it is marketed in Europe.
I've been riding the MP3 on the two-lane roads near Santa Rosa while visiting my son. They are bumpy and rough. The roads in Texas, where I was last fall, were full of pot holes, so I got in some swerving practice. I've been practicing due to the those two-wheels up front, because I wasn't sure of them at first. They work independently of each other up and down. Counter steering works well on this scooter. It's ride is a little rougher than that of the Silverwing.
The MP3 is a very tall bike and has more weight up front and higher than the Silverwing. It is balanced more like a motorcycle. It slows down on the downhill grades a little more on its own than the Silvewing; I assume it is geared lower. It brakes well when bicycles cut in front of it. I'll get it out on the highway in Nevada where it isn't as crowded to test the power and how I like it on long trips. The Piaggio BV has the same engine as the MP3, but has one front wheel. It is also ess expensive than the MP3.
If you've never ridden a motorcycle before and are contemplating getting a larger scooter for longer rides, consider taking a motorcycle safety course. You're dealing with the same problems that motorcycles do. Once a teacher, always a teacher.
I love both of the scooters.