If we were to divide things into classes, it'd be like this:
1080p - Class 1
720p/1080i - Class 2
1080i isn't true 1080p because its simply two frames interleaved together to make an image and not a true 1920 x 1080 progressive broadcast.
Secondly, anywhere below 40-46", you don't even need 1080p. After that can you discern the difference, otherwise it'll look exactly the same.
And blu-ray isn't exactly a very good option for right now. Reason being that it still has some issues and the high price. After that will it start going mainstream, heck, look what happened to the DTV bill - it's gonna take a while.