Mainlines or Premiums?

Mainlines are cheaper!

Unless there's a premium casting body that looks super rad that I'd want, I'd rather have 6 mainlines than a single premium for the same price.

I can understand the motivation. In the beginning this was a non-issue because Mattel produced lots of metal-on-metal toy vehicles and sold them for $1. Plastic is cheaper and easier to produce so it's eventual preponderance in mainlines was inevitable. While racers value weight in the interest of kinetic power, weight is also commonly associated with monetary value. Mattel knows this, hence their use of advertising "Metal on Metal" on the more expensive premium packages. 

But to answer the original question, both. About 90% of the cars I put on the track are mainlines but I do have a modest fleet of wheel-swapped premiums.

Depends on weight class. Anything above maybe 80g and it's gonna start getting difficult to make weight with plastic parts

Premium cars will be less likely to flip on an open track thanks to a lower center of gravity

If you aren't into modifying them, the premiums usually will be slower due to the rubber wheels, especially on open tracks. in some cases they may not make it down the track at all. But if you are modifying them. they can be a great place to start, due to the weight as stock.

if I see a premium casting with race potential, I buy it for the race.

otherwise, I just buy lots of $1 mainlines

as was said above, the cast bottom helps get the CG down much lower

dr

High speed polish.

What I have found out over time is that the premium bodies work much better, but if it is a real rider, some of the mainline wheels are the best!

My favorite types of wheels are the ones that are big, and the rear wheels are slightly bigger than the front, and are about a sub centimeter in width. Like what are on the hot wheels green or red drones.

If you customize any Matchbox cars, or cars of a different brand than Hot wheels (Unless they have some amazing wheels, which is possible)I would REALLY recommend using some of those style wheels.