Playroom, it will get so much more use. We have a playroom and the DDs love it. I don't let my girls play upstairs (but they're only 1 and 4), so it is nice that they can play in their playroom and be around me on my days off work. I wouldn't want to not have a space downstairs that they can call their own.
It also means that then I can keep the living room area child free - so there are no toys in there, but with a 15ft playroom to play in, why would they need to?
Also, I find that toys do not get trashed, because I have got pleanty of storage for putting things away - plus the girls know that if a toy gets broken (either purposefully, or because they left it out and dd2 got to them), then my eldest knows the toy will go in the bin and will not be replaced. So, she respects her toys, and I'm downstairs too to keep an eye on her and to make sure she doesn't get all of her toys out!
So yes, go for a playroom! And as others have said, when the children get older it allows them to have a chill out room, where they can hang out with friends, without you having to put up with them in your front room, or having all the boys in their bedrooms!