haha..of course there are many difference...
your partner has to provide a lot of documents to prove that he's eligible to be your guarantor. and you will have to pay 800 euro for this ID card.
there is difference when you want to seperate. if you are married, you share 50% of your total assets unless you have pre-agreement
if you are just living together, holding a partnership ID, once you break up, you have to leave NL within a very short notice.