Will this be a production server? If not, go with whatever your engineers are strongest on, although I find that Windows "engineers" rarely know how to tune a machine regardless of what its running - and thats not exactly the slight it sounds like. Theres been a drift of responsibilties for guys who used to be network engineers to administrate big, mission critical Windows servers and managnig a box is such a different beast than managing a network. Especially on boxes that are hosting a database; the old shrink wrap mentalitiy doesnt work. Unix/AIX is great for informix and I have found that when it comes to knowing how to actually do things besides installs, fix things or tune things, unix admins to be light years ahead of their certified windows peers.
To be honest, while there are OS limitations and bugs to consider, once IDS is stable you really have to start thinking of informix as sort of a stand alone OS that happens to sits on top of whatever OS your hardware is running. Thats due to the virtualization of CPU and IO, etc...that IDS does.
