Deffered rendering gives you tons of lights with actual shadows being cast from them, FM5/FH22 only have shadows from the sun, headlight and lampposts don't cast anything, forward rendering is used mostly for transparencies, because deferred doesn't play well with it, but other than that it's mostly obsolete. Deffered rendering gives you much better lighting with good performance, but of course required a big frame buffer, which Xbone's eSRAM can't fit, and DDR3 is too slow to handle in 1080p. Not to mention that FM5/FH2 don't use any of the advanced modern techniques like screen space reflections or any kind of GI.
Again, there is no "this or that", as the quote from the DC devs shows.
And there is that from a banned site:
"Forza Horizon 2 is definitely a sweet looking game, and part of the reason for that visual glitz is in a rather new technology named Forward Plus Rendering, as explained today by Creative Director Ralph Fulton during a behind-closed-doors presentation at Gamescom.
According to Fulton Forza Horizon 2 is the first console game to use the tech, which allows it to display thousands of dynamic lights in real time, making the representation of cities like Nice possible.
In addition to this we also learn that the game has seven different radio stations, and over twice as many tracks as those included in the first Forza Horizon.
Finally, Fulton also had words of praise for the Drivatar feature, mentioning that the day in which they turned them on at the studio, racing in the game completely changed in front of their eyes.
Thats definitely promising, and I honestly cant wait to get my hands on the wheel again tomorrow."
Again, FH2 used forward+ and not only forward rendering:
"Digital Foundry: You're first to ship a console game with Forward+ rendering - what is it and what specific advantages does it bring to the table?
John Longcroft-Neal: The Forward+ technique we use is probably better described as 'Clustered Forward+'. All Forward+ techniques revolve around splitting up the screen into a regular grid of sub-rectangles (typically 32x32 pixels), then finding out which lights potentially effect each sub-rectangle before you start rendering. During surface shading, you load up the reduced light list for that sub-rectangle and process only those lights. The goal is to avoid processing lights that have no effect on the surfaces in the sub-rectangle.
The standard Forward+ technique uses a depth texture of the scene to cull lights from the list. There are two issues with this approach; firstly you need to render the depth texture as a pre-pass before the main scene in order to create the light lists; secondly semi-transparent surfaces cannot render to the depth pre-pass.
Clustered Forward+ avoids the need for a depth pre-pass altogether by calculating light lists at multiple depths for each sub-rectangle and using the most appropriate cluster during surface shading. We generate the light cluster data all on the GPU using Compute shaders and this is done for any rendered view that requires lights.
The advantage of Forward+ for us is that it just works with MSAA, at any level, whereas deferred techniques struggle to maintain decent anti-aliasing. Secondly you get the other benefits of forward shading such as allowing complex material types such as carbon fibre and car paint that are difficult to achieve using deferred techniques. We found that we could easily 'plug-in' Forward+ to the existing shaders which were already designed for forward rendering. The advantages of the Clustered approach to Forward+ for us were that semi-transparent surfaces did not need special consideration and most importantly we did not need to render a depth pre-pass."
http://www.eurogamer.net/articles/digitalfoundry-2014-the-making-of-forza-horizon-2
And it even features 4xMSAA which I don't know any other game of on current gen.
