Ok, bear with me here, bit of a long post but I said I would do this last night for a few people who wanted an unbiased screenshot comparison:
3 scenarios, 3 cars, 3 tracks. I tried to match the lighting and angles as best I could, I do realise some are off so dont go off on a tangent about it as I am aware some are wrong. Also, please note, these were done with each games "stock" photo settings. As you can see, GT5's stock settings have a few more adjustments applied whereas FM4's stock settings are untouched from what you see when you pause the game. So bear that in mind when comparing these also.
Tsukuba - Aston Martin DB9 - Sunshine
Shot 1
GT5
FM4
Ok so kicking off with a scenic shot of Tsukuba, firstly as you can see, although both are set in sun, it is coming from a different angle in each. Detail wise, its a close one to call, the major thing really is the tree at the back, not a very good looking tree in the FM4 shot, however Forza's grass looks better with GT5 edging it on road texture. I also personally feel Forza's lighting looks a little more real than GT5's.
Shot 2
GT5
FM4
Here we have the DB9 drifting beneath the Dunlop bridge. Again a simalair story here. The cars have very different appearances with the car in Forza looking a lot brighter, and the tyre smoke from GT5 looks a lot better than Forza's small puffs. The backgrounds here are very different, Forza has a lovely mountain vista whereas GT5, while it still has a mountain vista, its considerably smaller and in a very different position to the one in Forza. Small things like power-line direction and barrier colours also differ. Looking at real life shots, FM4 is closer in terms of where things are and colours of barriers etc.
Shot 3
GT5
FM4
We have here another shot more focussed on scenery. Looking at both pics, in terms of accuracy they are both pretty much spot on. Except for some differences in sponsors (xbox 360 etc) they are almost identical. Based on videos of the real circuit, GT5's pit wall is the correct colouring.
Lexus SC430 - Indianapolis - Sunshine
GT5
FM4
Okey dokey onto Indy. Again, both sunny and again its from differing angles. Kicking off with a view down the straight. The jagged shadows of GT5 very present here unfortunatly which gives the picture a big downer. A shame as it looks very good indeed. A massive downfall for Forza here is the road texture. Seemingly little effort has gone in to making it resemble the road surface as accurately as GT5 has. That said, although it is closer to the real one, its a strange shade of green.
Shot 2
GT5
FM4
Again, in these 2 shots very differing opinions on the road surface. Forza's is graphically nicer thanks to some low res textures on GT5. Again after looking at various photos of the real circuit both are pretty inaccurate, but this time Forza gets it closer. There is a distinct different in the tarmac between the oval and the GP circuit, and both games suffer from not differentiating them enough. Besides that, both games nail the scenery here.
Shot 3
GT5
FM4
Curse of the incorrect angles here so I apologise for that. GT5 looks a lot more photorealistic than Forza in this shot. Again both games look accurate although GT5's textures look a lot nicer in this shot than Forza's.
BMW Z4 - Sunset
Got the tracks wrong here as I thought GT5's Suzuka had sunset when it does not, dont know why I thought it did. So instead a lighting and car comparison.
Shot 1
GT5
FM4
Both games look very nice here. GT5 has a very blue tint in this shot however despite it being set to a sunset time. Although it is most likely to be down to the angle but whilst driving it was distinctly less sunset-y than Forza. The car models, both look good and are accurate. Forza's lighting again making it look a little better.
Shot 2
GT5
FM4
Tried to get a good look towards the sunset. Again Forza seems more accurate in how the time of day is portrayed, whilst GT5 does look good, it is not accurate in terms of the sunset lighting.
As you can see, in stock photomode form GT5 does look nicer. The next set of comparisons I will do will utilise the photomodes better with shots being adjusted for the best looking photos possible.
This is the key thing here because Forza's stock photos are just that, stock, whereas GT5 has some post processing even before you make your own adjustments. So look out this weekend for the "no holds barred" screenshot comparison this weekend along with a photo travel comparison.
Sorry for the long post, thanks for reading through.