Thanks for the suggestions Bob.

The price of the Sigma is actually quite ok here in Finland. I can get it for 2099€ (~2890$US) with a protector, a CPL filter and tripod collar + case included. I have considered the EF 300mm f/4.0L IS USM and 400mm f/5.6L USM primes and will rent at least one of them for testing. Sadly I can't rent the Sigma since the renting business is really small here.

The Canon primes cost both around 1400-1500€ (1900$US - 2100$US) here, the longer being cheaper, so the savings wouldn't be that great after buying filters and tripod collars compared to the Sigma. Same holds true for the 70-200mm f/2.8 II which is 2000€ here. And anything above 2500€ (3500$US) would be too much for my already strained budget so the really big guns are out of the question at the moment.

I think I would like the flexibility of the zoom in any case. Also I already have the 70-200mm f/4.0L IS USM for a light walkaround lens (one reason against the f/2.8 II version upgrade and the 70-300L) so the weight isn't such a problem with the Sigma. It also has weather sealing now and based on photozone's review the IQ seems to be there. Also it will take the extenders and can become a 240-600mm f/5.6 with the 2x which is quite a feat for the price.

Here is a review in Finnish and some photos with extenders: http://www.janneheimonen.net/Artikkelit/Sigma-120-300mm-f28-EX-DG-OS-APO-HSM-arvostelu/8/

If I end up buying it I will surely do some testing if you are interested.