Very, very good cars. The car is much more adjustable on the limit than the Zed, and it certainly makes a much better noise! Interior is of a substantially higher quality too as you'd expect, but as said above the running costs are substantially more: Brakes especially aren't cheap, a good friend of mine who owns an early SMG model was a tad shocked at the prices.
As far as upgrades go, the brakes are the big weak point and the vast majority of owners upgrade the calipers and discs so if you can get one that has been done even better, if not expect to pay £2K for a kit. Electrics can be seriously expensive if they go wrong, but they don't tend to. The SMG can be a bit temperamental (control boxes and pumps are known to fail), but it is a brilliant piece of kit and set to the most agressive mode an absolute hoot!
In pretty much every way that matters they're better than the Zed. But then they should be, seeing as how they were more expensive to buy.