Hi Tom,
The webserver type isn't really relevant to generating the certificate - for this provider asking the question, it might relate to the format that they output the certificate in (e.g. Windows formatted key vs. a PEM encoded). In the end, you want a PEM encoded certificate so, in theory, if you request 'apache', you should get the proper format. In the end, whatever format they send, you should be able to convert it to the PEM encoded certificate.
HTH,
Paula