A 3200mah battery would be charged at 3.2A at 1C charge rate. The 75w charger could charge your 3s battery at 75w/12.6v=5.9A. I used an 80w charger for a long time and it was fine for all my needs until I got a plane that takes a 6s battery. So unless you plan on parallel charging or getting really powerful planes soon, it'll probably work just fine for now.
To answer your charging time question, at 1c charge rate, you'd still be charging at 3.2A with either charger, so they will have similar charge times. But I think they will both be faster than the stock one. All batteries charged at 1c take about the same time to charge. When you have batteries that can charge at higher rates is when you can speed up the process.