The 09-onwards Vantage has the 420hp V8 as well as more torque. Straight line speed is really a toss up with the M3. Handling is great, suspension is great. I honestly think it will be very similar in feel to the M3 (coupe vs coupe). The Vantage roadster is a lot lighter than the e93 M3. The look and sound of the Vantage is simply the finest on the planet.
The only downside is that once you get into exotics (and to me an Aston is an exotic), service can become a real PITA when needed due to the paucity of dealerships.
Still, to own a car like this is the experience of a lifetime. I say go for it and don't look back. AM Vantage is high on my list for my ultimate car which will be my M3 replacement, likely in 2015 or 2016.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner