Burt: Where to begin......well here is what i think: I have been running a Deckel FP4NC since 92'. My machine has the dialog 4 control. My sense is that the machine is very good, better iron than most of the machines out there. The way they move (the "Y" axis not on the knee) is more rigid than a knee mill. The use of direct reading glass scales on all 3 slides gives an accurate move even with ball screw wear! The machines are built heavier and more rigid than most machines in their size range!
However they have some limitations: First if you are going to cut aluminum with small tools you will be somewhat hindered. The max spindle speed for the early FP machines is 3150 RPM. There are some that have a manual 2 range shift that allows the vertical spindle to run at 6300 RPM. Be prepaired, Deckel parts and service are very pricey....rebuilt logic cards for the dialog control cost $3500.00 or more exchange! The "Z" ballscrew is in use for all depth moves, so if the machine was used for peck drilling a million holes you may want to be sure the vertical slides and screw are in good shape!
I am not familiar with the 3m control other than looking over the programming manual, but i can say that it looks less user friendly than the Dialog series! You may want to see if the machine has an RS232 port for communication with a computer. Doing all your programs at the keyboard of the control gets old fast! I do know that the 3M will not do DNC (drip feed from a computer) so you are limited to the size of program that the memory of the control will hold! All that said, they are butiful machines, very versitle wit the 2 spindles and all. I wouldn't trade my FP4NC for a truck load of Bridgeports, Fadels, of Haws...(this will most likely start some trouble) But for me the Deckel just works! Good luck, visit the Deckel group for more information....check out the post on the FP3NC toolholders there is a guy there that has the same machine that you are thinking of buying.
Ross