we are thinking of driving down to carveiro from france,what would be the quickest way? drive down spain and then along or go through the top part of spain then cut down through portugal,any advice please.....

We have done both. Cutting across diagonally from France to CVO is quicker, and probably cheaper and easier on tolls. Roads and toll system better in Spain. There are some great places to stay or just take a break on the way through. There are other threads on this site with recommendations.
The route across the top of Spain is prettier and if you are going to do that I would suggest you do so when you can take a few days to appreciate it.

I totally agree that this is a worthwhile and scenic route, both in Spain and Portugal.
The only drawbacks being the cost of petrol in Portugal, compared to Spain, and the fact that it will cost over 50 euros in Portuguese toll charges.