SPAN Plus offers a correlation meter (which Melda does not) but melda offers a sonogram view and more importantly Collisions view, which in theory should help you find potential mix problems. Also cool is the "Deharmonize" mode, which allows you to get rid of harmonics and focus on the fundamental frequencies of your sources.
It looks like SPAN plus limits you to viewing only 4 spectrums at once (source plus 3 inputs) where it seems MMultiAnalayzer lets you easily see all available analyzers and select how many, or how few, you want to see - there seems to be no limit.
if you need metering, span.