It would be very hard choice for me, both knives are absolutely favorite models and use extensively both of them... Both great designs, very strong, will take unbelievable amount of beating before give you issues.
However, I think that Rat1 is more suitable for outdoors because of thinner blade behind the cutting edge ( still plenty of thickness ) of both of those. IMO, blade geometry is better suited for processing small game,
and edge to grip angle is more suitable for working when kneeling or bending. It doesn't mean Griptillian won't do all those tasks, I just think the RAt1 will perform better. After all it was designed by Randal specifically for work in the jungle.
They used to have nice videos on the Randall's Adventure site, showing different uses of the Rat but for some reason I couldn't find it... Currently you can see all Rat1 models on the Ontarioknife dot com site.
Here is a quick picture of both ( I don't have other than Tanto blades on my Grips, sorry), seems to me the cutting edge of the Rat is little bit longer than on the regular, non-tanto models Griptilians.
There is another very good blade design, IMO much better suitable for outdoors, built by Ontario for Randal - the Avispa.
Unfortunately, it is a frame lock and it does have serious problem if you move the clip for tip up carry, the way I usually do it. The clip presses directly on the lockbar and opening the knife with one hand is impossible, at least in my case.
If it does not bother you to carry tip down and to have frame lock to work with wet, cold, dirty or greasy hand out there, it will probably be better blade geometry for outdoors, even with inferior steel, than the D2 on the newer Rat1 models...
This is what I'm talking about, Top - Rat1, old model, Taiwan Made, Aus8, the Avispa are the two knives in the middle, the one on the bottom is the smaller version, called Zancudo.
Here is the issue with the clip hitting directly over the lock bar when moved for tip up carry.
Since I like the knife but it's inconvenient for me to use it tip down or not to be able to open it with one hand, I simply found a clip from Benchmade and installed it for tip up.
It only fits on two holes but it holds very strong, I don't really care about it as long as the knife is functional. This way it worked just fine and I did use this knife quite awhile.
As many others suggested also, if you want a lock back/backlock knife, I personally wouldn't look for something else but Cold Steel model. I would offer any of the blade shapes the Voyager model offers, because I have the most experiance with those, but there are plenty other CS models that will be suitable for what you're looking for.