Don't forget to consider any services provided by the company. They might provide things like support, installation, customization, template design, or training. It can make a big help when you're a small, over-worked team.
For example, we recently moved our external website to a hosted solution. It's not the greatest platform, but it does what we need it to. Plus, it's a big relief not having to worry about the website. I still answer some questions, but it's mostly HTML/JavaScript stuff... as opposed to chasing bugs or whatever.
Honestly, I don't think you'll have any major problems with ASP, but--all else being equal--you'd do better with something more modern (especially when you think about having to maintain everything 3-5 years from now).
As for Windows/Linux and ASP.NET/PHP, I don't think you'll notice any huge differences. If your org is more comfortable with Microsoft products, Windows-based solutions are an easier sell.
I checked out the SchoolWires site and a live site that uses it, and it looks like a pretty straightforward CMS... comparable with the other major players.