Well I guess it comes down to personal preferences. To me Forza 3 has given me quite the racing experience, possibly the best one i've ever had, but we know GT, we know what Kazunori can do to a racing game, and we know that he can take it over the top again.
At this time it's just too soon to speculate on how big the improvement will be on GT5, but as a PD fan, I have the utmost faith on them. Come next year, iwe'll have both of the best racing games ever, and to me, and to each gamer, that's what really should matter in the end ;)
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"