I felt that the DS3 has been a significant improvement over the DS2.
The feedback seems to be a lot smoother with the DS3, this is useful especially over the kerbs of London.
Perhaps this is just me, but since the update and being able to lock wheels the DS3 feels nicer again, with patches of vibration as you cadence brake (add/remove pressure on the brakes to prevent wheel locking) allowing you to judge more accurately how close to lock up you are.
As others have also said, the extra weight of the DS3 also helps it to feel a little more comfortable in your hands. I always found that the SIXAXIS was too light and would "give" a little when making multiple inputs at the same time.
In my view DS3 is a fantastic investment and should have been what the PS3 shipped with in the first place! Just don't order from CD-WOW.com, i've just ordered a second DS3 from them. The first one arrived within 3 days, the second is taking ages, i've just received a 4th delay email stating "we will not be receiving stocks within the foreseeable future due to supply problems"
I'd recommend getting down to your local independent game shop and buying one from there it's worth that little extra (most are selling them for between £35 and £45)
Hope this helps!