Which operating system is more unstable Openfire for 1500 users?
Windows or Linux?
Unstable? Well, there are plenty of users complaining about issues either with Windows or Linux setups here. Though, i think more Windows issues (maybe because there are more Windows users). Personally i would go with Linux, because i use my Arch linux cmd only box for 7 years now without issues (well, i have only ~200 users).
I went the Windows route first and had a bunch of headaches. Ended up using 64-Bit Ubuntu w/ MySQL and it runs like a champ.