Recently did 4 nights at the LM Brussels (having stayed there several times before). Really, the location vs. the Sheraton is a no brainer. You can walk just about everywhere in the old part of the city from the LM. The Sheraton is a train/subway ride then walk from whatever attractions you want to see.
The front desk staff was pretty good. They gave me a junior suite upgrade, which was just a larger, corner room, on the 5th floor. They even somehow knew it was my birthday, and they sent up a bottle of champagne and some Wittamer chocolates. As for the internet, they have this weird thing where they charge you by the minute for up to 3 hours, then it's 25 euro/day iirc.
The concierge wasn't really helpful at all. It appears that the bell staff uses the same desk as the concierge. I never saw more than one actual concierge on duty at a time, and he was always busy. If he wasn't, he'd pawn you off onto one of the bellmen anyway.
My big complaint is the bed. It was BY FAR the hardest, most uncomfortable hotel bed that I've ever slept in. After two days of sleeping in it, my back actually hurt. Stretching, etc, didn't help. I ended up making a bed on the floor out of the couch cushions for the last two nights.
I'm as loyal as possible to SPG. But on my next trip to Brussels where I'm dropping 200-300 euro/night, I think I'll pass on SPG and stay again at Hotel Amigo.