That's the 2 sims i have atm and thinking about AMS too... because both are incomplete.
Rf2 has the best physics/ ffb in probably the most anti user friendly platform u can think of (new game menu structure coming in the future though, i can't wait to see what it brings), im not a hardcore simracer exactly so AC is not bad for me but i miss rf2 immersion and detail in car handling eg. AC chassis balance through corners and curbs is rather neutral vs rf2 which is the quite opposite , in general search for ISI gmotor engine and variables if u dont have such titles u should really look into rf2 or ams etc. for their physics
Rf2 AI think of a supercar without a steering wheel / sort of abandonware so keep that in mind, but if u plan to race in online leagues its a no brainer imho, dont expect to find online races in the company's servers in a breeze, they re not very busy
AC AI think of a decevo,i dont play online cant comment but i see there 're plenty of leagues for both games
Rf2 base content for cars available is very good nowadays, plenty of top quality cars in physics,sound (although with bugs included) and looks. for open wheelers its in the top imho - u can laugh with other sims in comparison, to even begin comparing with AC in general u need all the DLCs included except if ur looking for road cars and supercars then AC is better, mods can be very high quality as well and similar to official content level.
car interior better looking in AC
Rf2 track content is improving also with new paid (laser scanned) tracks and free ones as well but graphics quality... well theres no standard quality and its all over the place even among the "official" ones , actually tracks are made from various studios and associates whatever and lack in consistency while the graphics engine itself is not bad at all
AC on the other hand has better and much more consistent quality among tracks plenty of good looking official-like mods and the pp filter really delivers and ur not in the mercy of your nvidia driver (dont know about amd) which is configured for 10 yo doom players
AC is the only sim that has laser scanned tracks that are not very accurate/realistic , quite an achievement i d say but for most of non-pros like me its ok, just dont get overly excited with AC's "laser scanned" tracks
Sound quality is also hit or miss for both, AC more consistent but not always impressive RSS mods deliver in spades tho (only AC atm), rf2 i have my favourite cars as well which can be amazing really but with various bugs to spoil the fun, others sound like ur moms hoover or mario kart...
AC has no rain or day-night cycle, rf2 does
AC has live telemetry, rf2 in the works...
AC has championship builder, rf2 not even a save button but the new ui is about to bring changes / planned for Q1 2019