|My new system|
There were a lot of things for me to consider, mainly financial. For me to make the switch, I would have to buy a new lens or lenses to replace the Canon 17-55 f/2.8 IS, which served me a my main lens on the 40D. The Canon 1D series would have really hurt my bank account. The question was weather a 5D Mark I or 5D Mark II.
I decided on a 5D MK II. A Canon 5D mark I would have been probably all I ever needed for my photographic hobby. The thing that put me off it was that there are no technological advances comparing to my 40D - no sensor cleaning system, slower speed, and there is no ability to micro adjust the autofocus of lenses. Plus, I was offered a 5D mk II for a bargain price I could not refuse.
The problems with lenses were bigger. I didn't know what lenses to buy for the 5D mk II. I didn't know whether to go for primes only system or zooms as well. The full frame world was too alien to me. I had not thought it would required so much study to determine what lenses would suit me best. Of course going for all L lenses would be the best thing to desire, but the cost of them are ridiculous. Zooms like 24-70mm L alone would cost me the same as my 40D and Canon 17-55mm IS combined.
It took me a while asking all relevant people to find out about good old lenses canon ever made. I tried 17-40mm f4 L, 24-105mm f4 L, 28-70mm L, 50mm f1.8, 24 mm f2.8, 35mm f2, 50mm f1.4, 70-200mm f4 L, 70-200mm f2.8 L IS and even 100-400mm f4.5-5.6 L. All these lenses were purchased for bargain prices.
Canon 17-40mm L was not for me due to the maximum f stop being 4 and I don't need 17mm, as I rarely shoot landscape.
Canon 24-105mm f4 L - for the same maximum f-stop was not for me neither.
70-200mm f4 - although I quite like this lens I don't see myself dragging it anywhere.
70-200mm f2.8 IS - this was too expensive and bulky.
100-400mm f4.5-5.6 L was not for me as I can;t see myself using it for what I shoot.
I settled on 24mm f2.8, 35mm f2, 50mm f1.4, and 28-70mm f2.8 L. I still keep my 85 f1.8 as that is one of the best lenses I have ever used and this becomes even better on a full frame. All these lenses costed me the price of 40D and the 17-55mm f2.8 IS that I sold.
The full frame format is so much different from the crop system to me, the DOF control is better and offers so much more, the lenses behave differently and I am enjoying the experience so far. :)