We have both also, the 14 Limited 5th gen and a 19 GX460 Premium, I echo most others on here, both are great and are very similar in many ways, even the rear armrest is identical in both!
This being said pro/s cons of each (at a glance)
GX

ros
- vV8 is obviously much more fun to drive
- Visibility is much better than the 4runner
- Has multi zone climate control great for kids in the back who are cold / warm.
GX: Cons
- Stock (non Luxury) suspension is a little spongy, smoother ride but I prefer the 4Runner
- Non ML stereo is really biased to the front passengers, not much sound in the back (i hear this is also the case with the ML system but i digress)
- Ours has the real leather, and its really soft, we'll see how it holds up in the long term.
- No rear USB outlets... really lame since our 14 4Runner has them...
4Runner: Pros
- Stock stereo is pretty damn good (on Limiteds at least) all around.
- Interior is more usable, cup holders / center console etc are better laid out.
- Auto climate button changes the whole cabin's temp at once, i like this feature.
- Exterior design is really good, like how it looks.
- Leather appears to wear better than that of the GX.
4Runner: Cons
- Visibility is not bad, but not even close to as good as the GX
- 6 Cyl has plenty of power but you have to really get on it to move all that metal around, pedal commander helps a lot.
- Our 14 has the tiny nav screen (new ones don't have this issue)
There are probably more but these are the things i noticed, if I had to pick one, i would probably get the GX solely because of the V8 but we really like both regardless...