I agree with everything the OP posted. I just recently got GT5 and the Logitech G27. I vowed to never play GT it hour a steering wheel since the day I bought GT4 with the DF Pro. I played Prologue with the G25, but ran into financial problems which forced me to return the G25. With that said, for the last 2 weeks I've been playing Forza 4, and overall, I feel like Forza 4 has more bang for the buck than GT5. If the G27, worked with the Xbox 360 without the XCom link, GT5 would get little to no play time.
I love the livery editor in Forza. I'm not big on ricing out my cars with vinyls out the wazoo, but I always edited my cars to look like the ones I have/had IRL. I would always go and make the state plates for my 01 Stang GT, or my 97 Eclipse GST, or my current 04 Stang GT. It gave me that bit of realism like I was actually diving one of my cars. The online marketplace, and Auction House were excellent additions in the Forza series.
I feel like these additions made the Forza series stand out from GT. also I read somewhere that Kazanori did want to make a real world driving simulator and I hope a lot of these features will be present in GT6, and that we won't have to wait another 7 years in between games like we did from 4 to 5.