For the most part, there is no such thing as a 1080i Plasma. Plasma like LCD's are a Progressive display. So 480P, 720P or 1080P. 720P Plasma's aren't normally 720P either, but 768P. They accept a 720P or 1080i input and then the picture is displayed on on the HDTV in it's native resolution, which in your case is more then likely 768P.
A Blu-Ray is 1080P. Now it's normally better to scale down because your trowing out extra Data to get it down to 768P from 1080P. If your set at 720P, you have to scale UP to 768P adding Data that's not there as it's already been tossed out. Understand what I mean? Seeing any difference, well that's questionable. Try it either way and see what looks better to you. Either way, your HDTV has to scale the picture.