Kristen, as a camera freak I strongly recommend you keep both.
I have the X10 and the GRD III and I find that they complement each other well. The X10 makes a great travel camera as it is small, light, has a decent zoom range and really great full HD video. The GRD III, or IV, as it is based on the same body shape, slips into a pocket or purse and produces excellent images.
I personally prefer the image quality of the GRD, as it is a full 12mp camera and extremely sharp, but if you set the X10 to M-size jpeg, Fine compression, and DR400, the images are very pleasing as well. As you probably know, DR400 gives you a much higher dynamic range level, meaning that highlights are less harsh and blown, shadows will still have visible detail rather than being dark, and overall the images will look more 'natural'.
If you are in the Australian sun, which I am, I'd advise that you leave the EV dial at either 0 or -1/3. I find that the X10 has a tendency to underexpose when shooting in DR400 mode. I've been doing a lot of shooting with the X10 over the last few weeks, and I find that it does very well in indoor and more dimly lit situations, places that a lot of small sensor cameras will produce less than optimal results. I find that the X10 creates images with a very natural colour balance, even in artificial and mixed lighting.
I wander around with the X10 on a neckstrap and the lens extended. This means that if I want to take a picture, the camera will be ready with little fuss. It's not an instant powerup like a DSLR, but the benefit is that you don't have to twist the lens barrel every time you want to turn on the camera. Just half-press the shutter button for a few moments and the camera wakes up, which you will be able to feel. The X10 shutter is way more quiet than a DSLR which makes for good candids, although the response time is probably better with the new GRD IV, due to the dual AF system.
I'm a bit envious. I've heard a lot of good reports about the GRD IV but I've already got loads of cameras!