Apparently, there may be a problem with saving modified cars.
http://support.rockstargames.com/hc/en-us/articles/200218557-Vehicles-disappearing-from-the-GTA-V-garage-or-losing-upgrades
The missions in GTA V appear easier, since when you fail, you don't have to start from the beginning. There are many checkpoints in each mission. Having finished every GTA game, except the PSP and iPhone platforms, I like San Andreas the best. The combat and driving portions are relatively easy, but I died multiple times while trying to fly underneath bridges.
I am going to play slowly and wait for Arkham Origins next month. At least GTA V has tremendous play value. Last month, I got Ducktales Remastered, finished in a few hours, and got most of the medals.