The increase in resolution between 720 and 1080 will not blow you away, but it is noticeable. What will blow you away is better blacks, higher dynamic contrast and better color separation with the 1080P units. Where 720 trumps 1080 units is lumens, but if have some level of light control in the room, that’s a mute point. There are several excellent 1080P units for under 3K now. If you tend to upgrade frequently, you might as well just bite the bullet and go 1080 now. Screens do make a difference. If you will have ambient light, you’ll want a grey or silver screen. If not, a positive gain white screen is what you want. Positive gain grey/silver screens are good for both situations, but they come at a premium price and some are directional (meaning the projector must be either above or below the screen). One thing to consider is projectors are constantly getting better. My opinion is to buy all the screen you can afford as it will outlast many projectors. But in the end, your checking account balance will determine what you buy, and with the choices on the market today, you can be assured that whatever you buy, you will enjoy it immensely.
