Clyde, I did what you are thinking of doing and added an 08 F430 to the 575M before Christmas. Unfortunately the weather and commitments have meant I haven't diriven either since, and not back to back which I am really look forward to.
However, I am back to front on spec compared to most as my 575M is a 3 pedal and the F430 is F1. I only ever considered a 3 pedal 575M many moons ago, and it took me nearly two years to find and buy mine. I also never considered a 550 over the 3 pedal 575M. For the F430 I only wanted F1 mated with the mid engined V8 and that took less than a week after spotting the add.
Not sure about values but as mentioned the lower the numbers the better chance of retained value. So with the F430 buying the best one with the most desirable spec is key. CF race seats, CCM brakes etc will help and if numbers of 3 pedals are low that too. But I have to say to me the manual seems a little last generation when in the F430, especially since the F1 was so good by then. Only my opinion tho and easy for me as I have the choice depending on the mood.
Current Fcars: 360 Modena
Gone but not forgotten: 360 Spider, 575M, F430.
Other stuff: '71 Holden GTS Monaro, '80 BMW Alpina B9, '74 Porsche Targa, Caterham, MB CL500, Land Rover Discovery.
Last edited by 575er; 02-03-2015 at 12:12 AM.