I think the first thing you need to decide is do you want an internal ballistic solution. The G7's is far superior the the 1600 B and the terrapin doesn't have one at all. So that right there may make up your mind for you.
The G7 IS bullet specific, you add in the exact BC and weight, whereas with the 1600B there are only 7 ballistic tables to choose from so you have to find the one that fits best for your specific bullet/load combination. I imagine that is one of the reasons they only go to 875 yards, and this is likely too far if your load doesn't match up perfectly.
I have the 1600 and have only read reviews for the G7 but from what I have read the 1600 series is more accurate due to the smaller beam divergence and will reach just about as far as the G7. I have ranged out to 1900 yards a few times and to 1600 1700 yards pretty easily with my 1600. Again, I think the G7 may get a bit further but the accuracy may be in question. With the 1600 it isn't always easy to get a reading at those distances but when I do it is dead on.
I personally feel the 1600 is a better rangefinder only than the G7 but the all in one concept of the G7 surely brings another level of simplicity and functionality to the playing field. I think the G7 would be really hard to beat if you are staying in the 800-1000 yard range. Beyond that you may want to start thinking about the 1600 or terrapin and an iPod touch with Shooter or similar ballistic solution app.
Here are two links that should be a lot of help. The first is Shawn Carlocks review and thread and the second is one that Broz did and has some excellent comparisons.
Gunwerks G7 BR2 Rangefinder Review
My Field Test of The G7-BR2 Rangefinder