There is a very big difference between digital magnification and optical magnification. I am not trying to start an argument, I am just trying to clear up a very common mistake. It is very common not to understand the differences.
If you would like more information, just do a search and you can find many articles that describe the differences. In summary: Digital magnification gives you NO additional information ( ie data) while with optical magnification you get additional data.
Here is a "thought experiment" (or actually do this). Take a picture shot with a 50mm lens (normal FOV) and blow it up about 10 times. Compare it with the same picture taken with a 500mm lens (this will be about the same magnification, about 10 times normal ). Compare the part of the pictures that show the same area. The picture from the 500mm lens will have way better quality. That is the difference between Optical and digital magnification.

There are cameras with either formats that will give you great pictures, and it is an individual choice what is best for you, and I see many good suggestions and comments in the responses to this question. DX sensor and camera technology has become so good that I suspect for most people, they will not notice the difference.
Two additional "pros" for the FF. I may have missed it, but these may not have been mentioned. Since the FF sensor is larger, it will gather more light. (think, putting a bigger bucket out in a rain shower). Therefore, it should perform better in a low light situation.This is assuming that the electronics etc are similar. In other words, don't compare a 6 year old FF with a new DX, as the new DX will probably out perform the older FF. The second comment is that much of the discussion (DX vs FF) discusses the difference in "image magnification" ( really crop factor - see below). At the lower end of the lens focal length area, there is the opposite effect. With a DX sensor, you will lose some of your wide angle ( due to the crop factor). This means that a 20 mm lens on a DX will give you an angle of view like a 30mm lens on a FF. If you want a 20mm angle of view on a DX, you will need (about) a 13mm lens. This wider angle lens will have more issues with image quality & probably be more expensive. (although most DX lenses should be less expensive than FF lenses)

I do not mean any disrespect by this, but your statement that: "when using a 600mm lens on a DX body, it will give you an effective reach of 900mm" is a very common misconception when comparing cameras with different size sensors.

Many people have been confused and think that a DX body will make a 600mm lens have the magnification of a 900mm lens. This is NOT the case. Focal length DOES NOT CHANGE. With the smaller sensor (DX), you are capturing a smaller part of the image being viewed (AKA "crop factor" or angle of view). If when printing (or viewing) the photo, you increase the size up to the same size as the image when viewed with a larger sensor (FF), you will have a "magnified" image. But, this is a DIGITAL MAGNIFICATION not an optical magnification.
In no way does a dx sensor make a 600mm lens like a 900mm lens.
Here is a pretty good article by Ken Rockwell. http://www.kenrockwell.com/tech/crop-factor.htm

great pictures. I was wondering what your lens settings were. Also, how much (if any) adjustment to the photos did you do. thanks