Well I can't find many bad points.
Wonderful cities both, culture wherever you turn, a lot of people speaks English, they are generaly very warm and welcoming.
The only problem could be with the cost of life, which can be really high if you don't know the right places to purchase everyday things.
But this can be solved with time

The Italian language you find there is probably one of the closest to Standard Italian there is, but here in Italy dialects are widely spoken as you certainly know, and Tuscany is not different.
Firenze is bigger, a little louder, but astonishingly beautiful. You can see something new and beautiful even after months of living there. It's the Italian city I prefer above all. A diamond.
Siena is amazing, it takes your breath away with those unbelievable little streets leading to sweet piazzas where you find nice churches. The only thing is: do you like to walk uphill?
I don't really see as many cons as pros in this choice, maybe someone else has had worse experiences there, but I wouldn't swear on it. You are also not so far from "Le dolci colline del Chianti".
To me it's the closest you can get to Heaven on Earth.