I might be criticised here, but Id probably go for the BES250, it is cheaper, theyre both thermoblocks, both have the auto-purge feature, both have the same 15 bar pump... The only difference is that the 800ES apparently is semi-programmable - that is it will always deliver the same volume of water through your grounds. But I dont see this as a big issue as, I think most people here would say to watch and measure it yourself anyway!
But as youve said, it always comes down to having a good grinder and good, fresh beans. Also, look into getting the single walled baskets. Breville makes these. They will give you better and more authentic espresso, providing you use fresh beans. :)