I have an '04GT and like you, I love it. The new GT is, from all that I hear, faster, lighter, etc. etc. That being said, its a big piece of change to buy. I suppose it all comes down to what is important to you, money in the bank or the latest and greatest bike.
We have discussed this choice at my house for some time. My wife loves the feel and look of our '04 GT and thinks its more "sexy" than the new model. The weight of our '04 is not an issue to me 98% of the time. We also have a Suzuki DL 650 VStrom and we ride it for slow back roads and times when the brisk power and smoothness of the mighty K12 is not required. It was way cheaper to keep the '04K12GT and buy the Vstrom than to get the new K.
I really like the new GT but for me it just wasn't worth taking a beating on my low mileage '04 to replace it with a very expensive new K. We are happy with our '04 and it may well be the last new BMW we get.
By-the-way, one thing I do like about my '04: you can work on much of it yourself. These bikes have been around for a long time and there is a great knowledge base out there for the DIY folks. If you are into this it can save you a ton of money. Myself I enjoy doing the service work. After 40+ years in the automotive technical field I am OK with servicing machines. I do good work and only hit the dealer when special tools or equipment is required. When I do go to the dealer I know the tech, he knows my machine as he has worked on that model for years. He has seen it all and knows what to do and how to do it. That level of experience takes time to build up. I am sure that in a few more years people will figure out how to do some of the service work on the new GT also. Presently it appears that you must return it to the dealer for some very expensive basic service work. I am sure some of the new GT owners can comment better on this than I.
So, it you lust for a new bike and can cover the tab, go for the new one. If you are happy with the "old" GT, keep it and enjoy it. I had a K100RS before my current '04 GT. It was great machine and I see a lot of them still out there purring away with 100,000 mile on them. The brick is a seasoned bike that has been around and had most of its issues worked out. Nothing wrong with keeping it if it meets your needs.
Good luck and I am sure there are those out there that will tell you how much better the new GT is. To me its a Value issue, is the new bike worth the big chunk of cash it requires.