If Alonso's main focus is to get more money, and be the top dog, then leave.
If his main focus is to be able to have a chance at another title, i'd say stay.
unknown Honda engine variable.
unknown McLaren chasis variable (this year's car sucks...even with a Mercedes engine).
I predict it will take at least 2-3 years, and i'd guess more like 5, to get a McLaren Honda back on the top step of the podium.
Either way i believe his choice has already been made....just not announced.