Hmm. Overall I would probably rate San Andreas better as I think it has way more content than GTA 5. GTA 5 doesn't really have a main antagonist, the story I think is much shorter, the music is kinda bleh, I believe there is more variety in vehicles in San Andreas and you can't even hijack the trains in GTA 5. Also the airplane speed in this game seems really slow for some reason.
Overall GTA 5 will probably end up superior for me just because of the online aspect that they seem to be putting most of their effort towards, but from a single player perspective - I don't think its as good as San Andreas. I would say San Andreas's world is probably bigger as well but I'm not sure if you count GTA 5's total area where you can swim underwater which doesn't really interest me.