Right now Maemo 5 definitely runs nicer on N900 than Mer, but they have different purposes. Mer tries to build a freer, more mainline "style" distro than what comes from Nokia. It still drags in some non-free bits from Nokia though, for such things as battery. It will punt non free parts before Nokia. It also is designed to run on older Nokia internet tablets, which is nice if you have one of those.