As Mark says, first stop is your local club to see who is running what. Whichever car is more popular, go for that as you will get great support and set-up advice.
Supastox builds well and needs just a set of front springs to get the set-up the National guys use. Red is the most popular, with green and blue close behind. Start with red, if you want a bit more steering, go with green. See Chris Ashton's set-up sheet on the Schumacher site for a great starting point.
Mardave now builds OK, and you need to get some help on the front end to know what a good build looks like - smooth operation of axle blocks on king pins being the most important area to focus on. Using the kit stuff will get you a good base set-up, front springs and side damper fluid to tune to taste.
For the Mardave, also consider the Zen front axle blocks. Use the 2 deg items, set for negative camber, and give the wishbones 1 deg positive camber. This is as good as he double-spring system on the current cars and means the car is easier to set up.
For the Supastox, Contact 50 fronts and 40 rears and for the Mardave Contact 37 rear and 47 front work OK. Ascari body for the most even perfroamcne through the corner, Ferrari to give less turn in and more mid-corner (in my experience). Any LiPo over 5500mAh and the Reedy Mach II and Dynamite (DPS) motors work best in my experience.
Once you've decided on your car, come back on here for any further questions. HTH
