The only reason I'm very sceptical about this is that City for the first time in Guardiola's reign isn't a guaranteed winning project especially domestically now with the emergence of Liverpool. He spent ?315 million on the defence and all that has resulted in is an average backline that actually consits of one midfielder.
Kompany,Toure, Silva have all left and some could argue that they've made the same mistake as Barca have and up until this point have not replaced them properly. Aguero also has 1 more year left on his deal. Sane has now left too and they've got Torres as a replacement.
They were very hit and miss last season, losing the league by a points margin that members here would be going crazy about had Barca been that far behind. They lost in the F.A Cup to an average Arsenal side and continue to unimpress in the Champions League. We shall wait and see but i'm not entirely sure that a Messi that will need time to adapt to an entirely new langauge,culture,teammates etc is gonna necessarily be the instant hit some might expect.
At his age, it's almost as big a risk starting a completely fresh project than it is trusting in a Barca rebuild.