Hi Paul,
I first got a P50/1.7MM when I got my 167MT, I later sold it and got a P50/1.4MM and can only say that I can't really tell the difference. From looking at the lens test results the 1.4 performs slightly better at the widest apertures, below F2.8. Now that I have an RTS3 body as well I picked up another P50/1.7MM, the 1.4 is heavier (more glass) but the build quality on both is excellint in my opinion. Optically both are excellent and if you are not frquently shooting below F2.8 there is no optical advantage at all in having the 1.4, if anything it's larger front element is likely to make it more prone to flare. I would actually say that a 1.7MM has one slight advantage over a 1.4 AE in that it's aperture blades have a different profile giving a slightly more pleasing Bokeh, if you want to go for a 1.4 I would wait for an MM version, after all you would want a 1.4 for it's wide performance which suggests lots of "out of focus" parts in the compositions which you would want rendered as pleasing as possible. The differences are subtle, they're all great lenses, hope this is of some help!
Cheers,
Art.