I’ve asked webMethods many times this question. Solaris and Windows are the two primary platforms by customers.
They do not have a strong demand for Linux at this time although it is growing. Last time I checked which was back in August only a handful of customers were using it. I do suspect it will grow though. We are looking at it pretty hard ourselves. We are currently Solaris. But our ops group is pushing us to Linux.
To answer the overall question, webMethods is a lot of different products. All of their products are not fully supported on all of their platforms. The first step is to look at your current and future product list and make sure it is fully supported on the platforms you are evaluating.
Next I would talk with my IT operations folks and see where their preference and skillsets are. If you are a primary Windows shop, bringing in Solaris came be a difficult challenge. Sames goes if you are Unix and you want to bring in Windows.
webMethods has also published a number of performance papers comparing Solaris, Windows and Linux with different configurations.
As far as the OS wars go, my opinion ( and that’s all it is) is that Linux, Solaris and Windows will be the dominant platforms for server based OS’s.