I've had a bit of a re-think on these two games after really falling out of love with Forza 4. This past week I've spent a lot of time getting back into GT5 and when it comes down to it, out on the track GT5 is the better game for me.
I don't want to cover old ground about GT5's weak points. It has many and I agree with them all. The engine noise is atrocious for the most part, I hate the standard cars and low-res PS2 tracks. The menu system is archaic (but not as bad as some like to make out). Offline is quite dull, etc, etc......
Regardless of this, racing in GT5 feels more natural in my opinion. The driving model is more refined with little nuances that I don't see in Forza. Yes, Forza's tyre model is probably more sophisticated but overall GT5 just feels right. One of my big issues with Forza is how there seems to be too much emphasis on power oversteer, as if the game is tailored towards drifting. High-powered RWD cars will break into power oversteer far too easily in my opinion. For example, if I jump in a DTM car in Forza, driving it is like walking on eggshells. I know cars like this should be challenging but they don't feel as planted as a high-downforce, purpose built race car should. If I jump into a Super GT in GT5 (an equivalently specced car) it feels grippy and planted like I imagine a race car should feel. It's a pleasure to drive, but still challenging.
Graphically, GT5 has a much more realistic look to it. The colours and lighting are much more muted and natural. Forza can still have an over-saturated look to it, and I think they've really overdone the lighting model, to the point where it doesn't look quite right. I appreciate that Forza 4 runs at a locked 60fps and is graphically consistent throughout, but I only race premium cars and "premium" tracks in GT5, and it's on another level to Forza.
Another thing I dislike about Forza 4 is how it neglects some real basic things. For example in the online lobby there is no option for qualifying or free-run sessions, no text chat function, no tuning menus and no previous race results page. Pit-stops are more of an after thought, as is realistic tyre wear and fuel consumption. It's like long/endurance races don't exist in the Forza world (they were removed from the event list for no apparent reason).
The PI system in Forza doesn't really work. The fact that Turn 10 have adjusted the PI for certain leaderboard cars underlines this for me. Rather than dealing with certain cars, the whole PI system needs a rework. GT5's PP system isn't perfect, but I have much more faith in it. From my experience it does its job.
I'm not really looking to start any discussion on the above, I'm just posting my conclusions of the two games after owning and extensively playing both. I do worry about Turn 10's focus for the Forza series. They are happy to churn out car packs but seem reluctant to support and improve this game (for instance the horrendous lag online that has caused some of the group I race with to give up on the game). I realise that GT5 took far too long to develop, and was unfinished on release, but I do admire PD for sticking with it and releasing a number of updates that add functionality and improve the experience. They've also added the greatest circuit in the world (Spa) via DLC, which should show Turn 10 that if you pick good circuits and model them as faithfully as possible they will sell in good numbers.
Perhaps, in the end, a two year development cycle isn't long enough for a game like Forza? I'm getting bored of the game and it's deja vu feel. They obviously haven't had time to update Silverstone, nor update the Nurburgring GP circuit which still uses the wrong layout and lacks the new grandstand and rollercoaster on the start finish line. Then they remove the Le Mans Bugatti circuit for no apparent reason. Another example is the lack of open wheel cars, which suggests the physics engine has limitations.
It seems to me that they are churning out games to a two year timescale rather than genuinely pushing the series forward.