There are many 720P TVs that will accept 1080I signals and display "1080I" as the resolution leading people to believe they are watching 1080I. You're not. Your TV is taking 1080I and lowering it to 720P. If that's the case, you're better off sending 720P because that's what it will end up as.
If you have a real 1080P that can actually display 1080P, then no, as long as it has a good de-interlacer, there is 0 difference between 1080P and 1080I, they are the same resolution just transfered differently. However, if you can't afford a TV with a good de-interlacer, go for 1080P always.