I don't know what they changed when the model was "upgraded" from the Perception 150, but I tried a pair of those and never was able to get a good stereo acoustic guitar sound.....I'm now using a pair of AKG c1000's, bought used for about the same price as new 150's - they are vastly superior...
I got one the other week. Haven't used it a hell of a lot yet so I can't really comment. I'll be using it as a sinlge mic to record acoustic guitar...maybe use it sometimes in stereo with whatever else I have...CADm177, MCA SP1, Behringer C2's.
I haven't used either, but having studied the specs and frequency plots, I can say that the only difference between the Perception 150 and the Perception 170 is the pad. The 170 has a -20dB pad; the 150 has a -10dB pad.
So, unless you were planning to use the mic on a snare drum, which is probably just about the loudest source you'd typically use a mic like this on, you can probably get by with the model 150 and save yourself $20 or more. I believe the old Perception series (100, 150, 200, 400) has been discontinued, so the prices will be falling until the remaining stock has been moved out.