Being I haven't played Forza since the second game I'm unable to offer a personal opinion, but I have seen compelling arguments from both sides.
As for content, the lack of uniformity in the quality between cars (a big part of that being the lack of cockpits for all cars) is something that seriously takes away from the game for me. I'd personally prefer less cars if it meant the quality was the same between them.
Again, having not played Forza for a long time I am unable to comment about its options. When it comes to Gran Turismo however, the lack of options is the largest problem I have with the game. I've said it so many times before that people following this thread have probably become sick of me whining about it, but camera customisability is so important. I also play PC sims and the ability to adjust these settings make a world of difference when it comes to immersion, and this is sorely lacking from console racing games (every first person game should have FOV adjustment, but for racing games it's particularly important). The consoles are capable of it (there was a multi-display menu that allowed FOV adjustment in GT5, until it was moronically patched out) so there is no good reason not to have it.
Continuing on the lack of options theme, I have seen many people that want the ability to create a custom race event/championship. The next couple of gripes are from my knowledge of GT5 as I didn't buy GT6, but I bet they apply. No option to have qualifying sessions, no option for standing starts, no option to hand pick opponents and the list goes on and on. There are so many little, feasible options that would improve the game, that the platform is capable of but are omitted. Then you find out PD or Kaz has been squandering their/his time making sure the stars are accurate to real star maps, or been reworking the rendering (despite the fact that no one was complaining about it in the first place). There are so many heavily requested features and improvements to be made to the series, but PD/Kaz just ignores them and does their own thing. For the moment, I'm done with the series, or at least they are certainly not day one buys. I'll follow what's happening when GT7 is announced, but I personally don't have high hopes for it.