With the last two big rule changes the Brackley team hit it out of the park, so on a chances level, Mercedes is the place to be.
I guess there are three things important to a driver like alo: paycheck, amount of work outside the car and succes. With mclaren, the last two seasons, he had it easy in the first two categories, just the succes bit that's missing.
It would be fun to see Alo next to Ham again. But... also under consideration, Ham has been loyal to Mercedes for 18 years now (and vv) and alo never really showed that trade. After last season it would be chic from Mercedes to consentrate on Ham and don't get another title rival in the team straight away.