I think its fairly healthy to disagree sometimes in a marriage! Arguing your point can sometimes make you realise just WHY you feel so strongly about something, but often other issues come out from your OH which can surprise you or give you some insight into WHY they feel the way they do.
My DH is pretty laid-back. Which can be a bad thing! He tends to go along with a "Whatever, babe!" attitude, and it's only when we start arguing about something that he tells me what he's actually thinking! I sometimes get annoyed with him because he is not a very emotive/ cuddly person, but he does get emotive and cuddly after/ during a row/ heated discussion! It can, I think, reinforce your feelings for one another, even though you have to realise your differences. And all couples will have differences! You can't think the same thing all the time, and if you think your point of view is valid, of course you will speak up.
We argue quite a bit, although usually make up and feel better for it, although it has been known to drag on, as we are both stubborn! I would keep going till its sorted, but DH often flounces off to bed, which drives me CRACKERS!!! (and he knows it!) We once had a major blow-out (our first big row!) over George Michael, and whether he concealed his sexual orientation!! And traditionally Christmas isn't Christmas without the Tree Arguement (I say real, he says artificial. He says small, I say big. We always end up with a real tree that I think is too small and he thinks is too big!)
We've had a similar arguement, BTW! DH doesn't want the kids to feel left out at school, so thinks they should get the latest trainers etc, whereas I think its ridiculous to spend that kind of money on kids trainers! He says I'll be sending them off in clogs that I've carved myself, with an iron hoop to play with!! They aren't at school yet, so we shall see....!! If your inlaws keep giving them stuff, could you maybe try to balance it by doing a periodic clear out and encouraging them to choose toys etc to give to disadvantaged kids/ charity, so they think more about how lucky they are etc etc??