I'd take the direct option on UA as it's quicker, there is no transit and the 764 is OK.

I would avoid Brussels - a confused company that does not know what it's selling and their Y service is a bit grim - probably worse than UA but it's a longer flight with a transfer.

LH might be the nicest option but transfers in FRA are to be avoided if possible. It's the one airport in Europe that makes LHR look decent!