The main downer about 1156 is its lack of pcie lanes. The best you can get is 16x/4x and 8x/8x/4x. Keep in mind however that a 5870 only loses 2% frames average from an 8x slot. This should be irrelevant to you though.
For an encoding system i'd take a look at my apps. If you can use the 6 cores the 1055T is definitely worth it. If not the i5 is going to be faster by a significant margin.
Damn there is never a clear cut winner in the same price range huh and yep the pcie lanes are what i was getting at. Premier cs4 and cs5 moreso take full advantage of multiple cores, but protools is stuck in the dark ages. Many people are still using 10 year old athlon 64's as they seem to provide the most stable system, however are just to slow to do anything else.
It all comes down to the platform discussion which is getting somewhat boring nowadays.. The AM3 platform seems to've a better future than the LGA 1156 (may be even the 1366).. You are bound to get better releases on the AM3 socket and moreover, the overall platform cost seems to be lower for the performance they provide.. While i say this, I'll recommend you pick a motherboard from the recent 8XX chipset series if you plan on getting the 1055T..