Hi Swissbiker,
I've been using Smart Pies since they first came out. I'm on my 3rd unit; I first started with the original SP with an external controller, then switched to the SP with an internal controller earlier this year. I bought the SP4 (vector) last month and have logged about 350 km on it. As a user of both the new and old unit, I can say the following in review:
Weight: The original and new SP are nearly the same weight. According to Gary Salo of GM Canada, the new controller is much heavier , but when I pick up both wheels, they feel pretty much the same. The controller makes up only a small portion of the overall weight, so you really won't notice the difference. It's probably a matter of like 100-200 grams more at most.
power: The new Smart Pie feels slightly more powerful than the old one. I keep the settings the same as my old motor, 48 volts with 30ah continuous, 40ah max. On the flats, I can cruise at up to 45 km/h with very light pedal assist. Even on the hills, I can keep it above 35. The biggest difference I notice is how smooth and quiet the new controller is. It's virtually silent; the gears and chain from pedaling will easily out-due any sound from the motor. The regen brakes are also MUCH stronger, strong enough to push you forward off your seat if you set it to max. I had to tune it back because of this, and because it kept tripping my battery's EMS. Just be aware of this.
Dimensions: They're the exact same as the old SP; the only difference is GM upgraded the wiring harness and plug points to be more air tight/water resistant. I normally wrap all my wires in electrical tape anyways to be extra cautious since I ride every day.
Overall: I love the Smart Pie; it fulfills all the needs of an e-bike, while staying nice and stealthy. The motor can really be hidden, and with the new vector technology, the public won't notice at all. I really don't see the need for the Magic Pie unless you live in an extremely hilly city like San Fran and need some serious torque. Speed wise, the SP can match the MP.