Your decision should be based on 1) are the cars easy to tune to the track, fun to race, fairly equal and easy to manage tech, 2) easy to acquire (lots of inventory out there, cars come up for sale on eBay, not too expensive, etc,, 3) robustness in racing situations (can easily last a whole race and take the abuse without breaking or being rendered noncompetitive, bodies don't come off in a crash), 4) have bodies available for the classes you want to run and a variety of bodies are competitive within any class.
By keeping the rules simple and stable, you can build a good base of racers that will keep coming back around any chassis that meet the above criteria. Heck, most every region with a healthy racer base has a "stock" T-jet class and those cars have not been made since the early 70's, which seems to violate rule number 2 but, because of their large base, they do come up on eBay.
Any magnet car class can get out of hand pretty quickly when someone slips in a set of modern "super" magnets so be very careful to not let that get out of hand.
I love racing most kinds of HO cars and we have some very healthy racing programs up and down the I-25 corridor around Denver. The club with the absolutely stable rules set (6 or so classes) seems to attract and keep the most faithful racers. There are track records that have not been broken in 5 years or more! I know that factor kept me coming back for every race!
Just my 40% of a nickel.
During my six years of Retro racing, my aspiration to be a mediocre racer has come and gone; my peers now assure me I have peaked (some time ago)!
The only thing I know about slot cars is if I had a good time when I leave the building! I can count the times I didn't on one hand!
Entitlement: The notion that one can have their slot car racing and EAT IT, too!
Slot Car Speedway and Hobbies, Longmont, CO, Home of the 155' Hillclimb track featuring the THUNDER-DONUT - "Two men enter; one man leaves!"