I think we have all reached a point in this debate, were we are all starting to realize that the argument of GT vs FORZA base on objective criteria simply doesnt work.
We have seen over the years fans of both series trying to use GRAPHICS / PHYSICS / CONTENT, as yard sticks for overall superiority, the logic goes which ever game has the best (or most) of these qualities surely is the better game......but theres a problem with this argument !
Lets hypothetically say FM4 has "better" or more "photo realistic" graphics than GT5, would that really sway most GT fans over to the Forza franchise ??? (i seriously doubt it), imo it wouldnt really matter, given that both games are so close in terms of graphical quality it renders the argument mute.
Those fans that historically have touted GT graphical prowess over FM would simply move on to a different feature to argue there case, because thats not really why they prefer GT over FM!
The same is true for physics, if FM4's physics engine turns out to be slightly less realistic than GT5's, that wouldnt really sway the average forza fan over to GT would it !, again Forza fans would argue their case buy simply picking a different area were they know Forza has a objective advantage.
So it must be clear to most rational people by now, that the reason why we so passionately choose to side with one franchise over the other really comes down to "subjective experience", objective things like photo realisim or superior physics are not our primary reasons for choosing our preferred franchise.
THE REAL QUESTION:
So we need to shift the argument into an area that isnt so prone to our own subjective criteria, for that we really need to look at this the same way the two developers will be measuring their games against each other.
The only real way to do this is to ask which franchise has the best business model, or rather which franchise is not only in the better position presently but also for the future.
For me Turn 10 has the better approach than PD, we have seen Turn 10 consistently closing the gap in sales between the two franchises, every release Turn 10 gets stronger, the same cannot be said of PD.
PD imo is hampered by the ivory tower they work within, with seemingly very little care for the needs/wants of their consumers or the gaming community in general, PD are working towards KAZ's vision for the ultimate car game, and are hoping that the strengh of the end product will attract a large audience.
Turn 10 on the other hand seem to be very market sensitive, their vision is simple, become the dominant (or best selling) car game on the market, to that end they are far more open to attracting a far broader spectrum of gamers than PD, They are far more attentive to the needs and wants of their customers, whilst consistently trying to broaden the appeal of the franchise whilst maintaining the integrity of its simulation roots.
The Future imo looks better for Turn 10 than it does for PD, Turn 10 are clearly out to win this battle of the franchises, by delivering the goods, listening to their customers, and seeking out new ways to broaden the games appeal to a wider audience.
If turn 10 can maintain their sales growth, is not a question of "IF" forza beats GT, its a question of "when" will Forza beat the GT series.