I have a TI15 and have been reading about the PDC. I know I can't buy it in the USA and the PDC is 18v while the TI15 is 15v. What are the functional differences?
Well, the main difference is that the Ti15 is an impact driver and the PDC is a drill with a four-gear gear box and a little bit of hammer action in it (of the non-SDS variety).
So if you need to screw in long lagbolts all day long you are better off with the impact driver as its easier on your wrists, but for everything else the PDC trumps the Ti15. Haven't touched mine since I got my PDC.