1. Where in Boston?
2. Are you taking mh/trailer?
3. Avoid 290 if you are towing or have MH. Cuts right through Worcester, it's somewhat narrow and winding, saves very little in tolls or time.
4. 84 doesn't "turn into" I90. It ends at I90. You can take I90 (Mass Pike) straight into Boston from there, but your ultimate destination will help decide what's the best route.
5. Get an EZ pass. It will save time and money as the tolls are discounted. We are now starting to go to a completely electronic system.
6. Decide if the extra fuel consumption and time is more valuable than paying tolls.