They're really two very different beasts. Just in terms of aircraft, the 767 is HUGE compared to the 737s, and certainly feels like it! Do you feel like learning how to fly a big aircraft, learning all the techniques of landing heavy-weights, flying passengers across continenets or thousands of miles away? Or would you rather something that almost becomes an extension of your arm, easily landable, offering plenty of short trips?
As far as the models these companies offer, you won't go wrong with either one, but both have a steep learning curve. I don't think that I could make a fair assesment of LDS's learning curve, having the PMDG as a background. Be warned, they are VERY different from any default aircraft. It will take plenty of reading and several tries before you even get the enginges turned on!, let alone all of the other systems necessary to make a smooth flight.
The FMC is something new for you to learn and they are nearly identical in both aircraft. They both require data entry (i.e. payload, gross weight, position, manual entry of the route, etc.), and a pretty good feel for the interaction between these and the different stages of flight / autopilot. This usually takes the longest to learn, but you'll find dozens of great tutorials and walkthroughs to use with it.
As far as the main panels and VC, LDS 767 wins the prize on this one. I see a HUGE improvement in framerates when I'm using the 767 vs PMDG's. The VC is also very smooth, even offering window reflections and a working windshield wiper! However, as far as the instumentation itself, the 737 certianly looks more appealing. The switches, gauges, everything feels more intimate and easier to read. Becoming familiar with it all certianly takes less time than that of a 767. You'll find that the 767's cockpit is far more crowded, harder to read, and a bit more challenging to get comfortable with! So in that sense, PMDG gets my vote, frame rates or not.
When it comes to actually handling the aircraft, LDS wins without a question on my part. It certainly is more challenging since it's obviously a very large/heavy aircraft (therefore landing certainly isn't as easy!), but the smooth taxing, the steady controlled turns, all those things a bout it make this a wonderful aircraft! The PMDG is certainly more touchy in that regard.
I wish I could just say "because of the better frame rates and nicer visual model, ground/flight crew interactions, and smoother flight controls, go with the LDS 767" or "because of it's nicer layout, more intimate cockpit, and a more practical range, go with the PMDG 737", but it's not that easy. LDS certainly has better ratings in my book, but I almost feel it that had to be earned.
Well, goodluck! and whatever you choose, enjoy the learning curve ahead....muahahaha....