Both OS are really steady and compatible with Oracle Apps....
But if I'm in your position I would have gone with Solaris box....(visit metalink)
to know in details why and what for I'm saying so
I read a lot of oracle documentation about it...
I'm going to install Oracle App Server in a Windows 2003 for development.
But I have a meeting in which I have to recomend a unix platform for production, just wanted to ask to people with experience like you.
For what I read Sun Solaris is the best choice... and so you think...