I had the opportunity to own the base 1.40 and now I have the ultimate 1.60. One thing I noticed is that the 1.60 has smoother HD playback (including from Youtube and Vimeo). The 1.40 HD playback is good enough for a lot of people and it doesn't chop that much, but for videophiles, the 1.60 is the way to go.
I also had the chance to edit in Final Cut Express 4 on the slower 1.40, and you know what, it's not bad, I didn't notice lag during editing and it was buttery smooth.
I hope this helps.