windows 8.1 or windows 7

It's your choice, both are perfectly fine OSes. As was mentioned earlier in the thread, Win 10 is slated to be released later this year and will be a free upgrade from either Win 7 or Win 8.1.

I believe you will find a wider selection of computers with Win 8.1 installed than Win 7.

At any rate, don't shy away from buying a computer you like simply because it has Win 8.1 on it.