Well, I have a CXS, PDC18 and a Ti15 and the thing Festool brings to drilling is the excellent Centrotec quick head/bit and chuck change system that works across drills and proper corner chucks for those situationa where you cannot fit otherwise.
The CXS is my goto driver, but if I need more umph I can just detach what ever tool I'm using and click it into the PDC in a blink and get it done.
In my humble opinion the Ti15 impact is the worst driver that Festool makes and definitely isn't worth the money they ask for it (I got mine @ 50% off so its good for that money). Impacts are so situational tools anyway that I don't really see their value.
Also the Festool drill bits and Centrotec assessories are of great quality.