I've got both. I raced a Blast for a year then moved to a Vantage. They are quite different: the shocks on the Vantage are much longer, the chassis is narrower (and so is the body), the chassis design itself is different - the front end sub-section bolts on rather than the rear... and more.
Some components may be the same or similar (like the slipper assembly) but overall they are two different cars, albeit built on the same principle of being affordable.
I prefer the Vantage, even though at first glance it doesn't look as top-notch as the Blast in terms of fit and finish. The thing that I ended up hating with the Blast was the 'joint' between the slipper assembly and the rear box (which is on a seperate chassis section).
Something happened to knock this out of kilter and it was impossible to put right. The Vantage, as I said before, has the back end as part of the overall chassis plate so this problem is less likely to arise.
The best person to ask is Nigel Reeks at freeprawnracing -
www.freeprawnracing.co.uk He sells both.