Hello. I went from an 04 GT JTD to an 07 GT Blackline. Only reasons were because the Blackline had more "goodies" (Bose, Leather, 18" wheels, Parking sensors) which I always wished I had on my 04 model, especially the parking sensors! However, if my 04 model had come with all the things I wanted, like yours seems to do, I would still be driving it now. Other than the add-ons, the differences between the 2 models:
Slightly different stereo, newer one plays MP3s
The cowls over the speedo etc. are shorter (although i prefer the longer ones tbh, my other half couldn't see the speedo
The little speaker thing under the handbrake in my 04 model in the pen-tray that I believe adjusts the stereo volume according to ambient noise is gone.
The interior light in the front is a newer design using clearer plastic. The on-board computer menu has a "menu off" option to cancel out of it, i seem to remember the older one didn't, or i couldn't find it anyway.
You can use the key fob to open the windows remotely but not close them like I could in my 04 model (very annoying, something to do with health and safety - pah).
Other than those I haven't found anything different so if I were you I would stick with what you have - sounds like a great car. If it hasn't given you any trouble and you have looked after it and you don't lust after anything the newer models give you (can't imagine so if they are lower spec) then i'd stick with your Bella.
Good luck whatever you decide.