It's common for only one of the two cars to be good. MR cars tend to be good for Gr.4, but tend to wear the rears more in Gr.3. FR cars tend to be good for Gr.3, with the caveat of needing more careful throttle control than MR, but they tend to wear the fronts more in Gr.4. There are manufacturers that are exceptions to these trends, though - the Gr.4 AMG has balanced tyre wear, and the Gr.3 Porsche is good for tyre wear. That is why these manufacturers are popular, making it hard to get into the top 10. Many drivers in the 40-50k DR range pick these cars because they prioritise getting the S rating by getting into the region top 200, and are happy to have no chance of getting into the top 10 for their manufacturer. At lower DR, e.g. 25k, pretty much the only way to get S rated is to pick a weak manufacturer, accept that you've no chance of making the region top 200, but have a chance of the manufacturer top 10. At the highest DRs, they are certain to be in the top 200 and top 10 no matter what, and their concern might be whether or not they'll get into the top 16 superstars race.
Then you have cars that can be good for some tracks but not others. If you go with the Megane Trophy Gr.4, for example, you'll be happy when Brands Hatch or Tsukuba come up, but not so happy when it's a race at Tokyo. Or a car can be good or bad depending on race settings. The Gr.4 BMW M4 guzzles fuel, for example, which is an advantage for qualifying as you can burn more fuel, and an advantage for races where the fuel multiplier means there's no need to refuel or save fuel, but a disadvantage when fuel is a limiter. Or a car with heavy tyre wear could be strong for qualifying where tyre wear doesn't matter that much, but weak for the race.
So you have to decide what you're trying to achieve, and look at the race calendar to see how many Gr.3 races there are vs Gr.4, and what the tracks and settings are to work out which manufacturers will be good for each race. Or you can just do what a huge number of people do, and pick AMG as a manufacturer that it's hard to go too far wrong with.