I just recently purchased a Littmann Lightweight and I adore it, but I have noticed it is shorter than my previous stethoscope and the ear pieces are smaller. In my case, that was what I wanted, but in some people's cases it may be a bad thing. I have very little ears and a normal stethoscope size hurts them after a full day.
What specifically did you want to do with it? Another point is I was just looking for something for physical assessment and the basics. The Lightweight is a good choice for that, in my opinion.
I am a little hard of hearing, when I first graduated I got the Littmann classic, then recently switched to the Lighweight. I have found that I can't hear nearly as well with the Lightweight as I could with the classic. I asked some of my co-workers and they agreed that they couldn't hear as well with the Lighweight either, so now I'm back to using the classic until I find a replacement. Im looking into getting the littmann master cardiology. Like I mentioned before, I am a little hard of hearing, and work in ICU, so I need something that can really help me hear lung and heart sounds well. I am not familiar with the MDF 767x, but I do know that them majority of my co-workers stick with the Littmann brand. The Lightweight might work well for you, you should try to find some online reviews that compare the two.