I might as well join in since I play both (and enjoy both).
Graphics
GT5 has the highest highs and the lowest lows for me.. the odd fleeting seconds of brilliance is offset by some pretty ropey textures and lack of track/scenery detail.. However that's all forgiveable, it's the screen tearing and obvious graphics over a rock solid 60fps that is the nail in the coffin for me
FM4 suffers from dropping LOD and AA to maintain a steady 60fps and it's graphical bar is defintely placed much lower then GT5 for spot comparison, however it seems that rather then blow the graphics budget on the cars, they seem to have spread it around, so tracks/scenery seem more often then not really well balanced for a good effect.
I really find the AA poor in FM4 when in most races, it's so much cleaner when hotlapping, and whilst I commend them for putting framerate ahead of AA, I would gladly sacrifice other graphical things to maintain 4xMSAA.
Physics
GT5 has a decent enough set of high level physics, by which I mean the general feel of the car is good, and the track interaction/feedback is excellent, it's certainly enough to fool most into thinking it's top notch. Where it falls down for me is that it lacks some obvious drivetrain/tyre physics that actually detract massively from the actual driving. For example, wheel spin away in an FWD or RWD car and it just tracks perfectly.. Then you realise that actually the FFB doesn't seem to have much coming from the actual tyres, the feeling of grip/slip in FWD cars is rather poor, and clearly you can tell by the tuning options that tyre deformation, pressure, width, etc are just rather absent. I love casually driving in GT5, but I hate hotlapping because when you want the feedback of what the tyres are doing in relation to the tyre physics, it's rather vacant
Forza is the other way around almost, it has quite indepth tyre physics, pressure, width, height etc are all there, and the grip/slip and torque behaviour is all there in the feedback making it great for hotlapping (for me at least). Where it falls down is that the high level general feel is decent enough and has improved in each iteration, but it's still not quite at GT5's level. And of course, the 'steering aids' and other little tweaks are quite understandable and rather overplayed IMO, but I can't figure out why they don't just mask them (as I suspect similar tweaks are in GT).
Sound
GT5 has a few decent sounding cars, but overall, Forza just has decent audio 90% of the time, it's much more consistent.. I can honestly say that as much as I like GT, the number of vacuum cleaners employed in the majority of cars is terrible.
Gameplay/Presentation
Another area FM has the more consistent and cohesive menu/gameplay by a country mile.. But it's career is really 1 dimensional.. very little variety and what variety there is proves to be really limited. GT has the poorest menu system by a mile, but it has more variety which is almost offset by the pointless game mechanics of servicing and having to earn paint chips..
Customisation/Tuning
GT is OK, but Forza obviously has made the most progress in this field (and probably explains some graphics issues), the ability to make your car your own is great, the first thing I do with a car is slap on personalised plates and make a car my own... in GT, the drastic lack of customisation keeps the game at a distance, I don't feel like I 'own' the cars in the same way.
Oddly, I would say that on balance, both are great games, and I stick by addage that GT has the highest highs and lowest lows with Forza being far more consistent in all areas which makes it IMO the better game, which is reflected by the vast majority of reviews, so I see no reason to post a million images and claim that somehow that negates the rest of the games foibles...