A media streamer is cheaper, but Im not sure Im convinced with that arguement completely.
To be fair - I run WHS and have a media PC in my lounge, so I haven't looked at media streamers at all, so Im out of touch..
For the OP - if all you want to do is stream movies, then a streamer may be the best way to go if you already have a NAS..
I have a media PC that has completely replaced everything else in my house, it is my stereo, blu-ray/DVD player, photo's, fileserver etc
A cheap AMD board, low power quad, and a small HDD for the OS, and you are sorted, running Media Portal, you can have the box as your media server, and we have the clients installed on all our laptops, and can watch TV anywhere in the house.
Where that arguement would fall down is if you have multiple TV's, we only have the one, if you had another TV in the bedroom, then you would want a media streamer anyway, so it may not be the best set up for you.
Media servers are cheaper, but I enjoy the flexibility that having a dedicated setup provides me with..