I don't play Forza a whole lot, but I have racked up some hours from Forza 3 onwards. Here's my sum up of the aspects of the series. The mainstream Forza games are simulation based, closed course only and modeled after real race tracks all across the world. No night/day cycles in those as of yet. Customization is tailored to performance rather than looks, although there are a few customization options depending on the car. Overall they are good games for the serious racer to compete in. However if you look towards Forza Horizon, entirely different beast. Free roam, full day and night cycle with rain which changes the way the car handles. These games are less simulation based, but actually provide a very good balance between realistic/arcade handling. Good for those care enthusiasts who just love to drive. Overall I'd say the Forza series is worth a try if you love cars and racing regardless. I can't speak for its difference to GTA5 racing, but since I'm sure those games have arcade handling, it'll take a while to get used to the simulation handling of Forza's cars. You shouldn't approach driving them in the same way you would GTA, or you'll likely end up t-boning a wall.

Forza 5 is a more simulation type game that's circuit racing. Forza Horizon 2 is more of an arcade type game that is open world. Forza 5 & Horizon 2 both have plenty of customisation. Everything you could wish to upgrade. Forza 2 has weather effects and day and night. If I was you I would wait and buy Forza 6 which is out in September. However, both Forza 5 and Horizon 2 are great games, FH2 is lots of fun.

Forza 5 is a more simulation type game that's circuit racing. Forza Horizon 2 is more of an arcade type game that is open world. Forza 5 & Horizon 2 both have plenty of customisation. Everything you could wish to upgrade. Forza 2 has weather effects and day and night. If I was you I would wait and buy Forza 6 which is out in September. However, both Forza 5 and Horizon 2 are great games, FH2 is lots of fun.