It would be nice if we could keep the MT talk out of either this thread or a Forza thread, if it's the FUD-based bullshit that was prevalent previously and would also be nice to differentiate between true microtransactions (buying tokens in FM or credits in GT) and DLC along the lines of car packs for FM.
Having put a few hours into Forza so far and finishing about 60 races to far and hitting level 30, I have a car for pretty much every series outside of old F1, Indy and hyper cars but still have over 4 million credits. I can quite literally play the entire career in the game without spending a cent on microtransactions or DLC (since the DLC cars aren't any better than anything in the 200 games on disc, just allow you to race things you may like from a personal point of view) and I'll be able to do the same thing in GT6 but I won't have to pay for any DLC cars they release most likely.
GT6 is definitely less graphically impressive than FM5 and there are still a lot of non-premium cars in the 1200+ where the quality is even worse, it seems that Bathurst is mapped a little more accurately than in FM5 and I think the sim handling is just a little better on FM5 due to the physics running a little faster (I think it's physics engine is running 60fps vs GT6 at 30fps). Both will have a shitload of time put into them though for me and I'll have to do so before I could feel comfortable saying one is significantly better in some aspect or the other (out side of graphics, and I feel at this stage I prefer the more consistant AI of GT6 than the drivatars in FM5 and their cheating), I wouldn't say either one should sell a system if you already have the other system but if you love driving either one will bring you enjoyment and having both will just be gravy....though switching between the two may affect your improvement on either as you have to readjust between the differences in the handling and physics of them