Road Runner 72;
Here is my perception of both those manufactures.
Racing Champions is considered a budget line vehicle with simple details. As you show, they often (normally) have die-cast bodies which by two large slotted lugs allow the body to be removed from the cars plastic chassis. I find the detail under the body lacking much definition and it's often out of scale. They seem to offer vehicles at times that are not offered by anyone else. Normally, if you shop around you should be able to find Racing Champions in 1:24th for well under $20.00
Action is a step above Racing Champions. The company Action is also RCCA, which also bought the rights from Revell in this scale for NASCAR and NHRA. These cars usually offer opening doors, hood, and trunk and much more detail under the hood and in the interior.
Both of these vendors use the tampo process to apply painted graphics versus decals. In every case I have seen, the Action's graphics are much crisper and thorough compared to Racing Champions.
In the USA, you can normally find Racing Champions at many of the discount department stores while Action is normally reserved to specialty vendors and hobby shops.
As far as purchasing a funny car and NASCAR; I recommend the most recent editions because each year they tend to add better detail. I would go with the Action "Elite" editions in 1:24th or try to get a 1:18th car. For the funny car go with Action and if possible shop E-Bay or the local shops for a 1:16th version (the detail is incredible). If you look hard, E-Bay can have them for under $50.00.
Happy Hunting and ...... HOPE THIS HELPS!