If you haven't done so, take a look at my Albacore project. it's a suite of rake tasks for building .NET solutions and will make working with Rake much easier for .NET solutions. You can get all the info you need from http://albacorebuild.net
having said that - i obviously have my own bias and preferences for which tool i would rather use. so take this with a grain of salt and understanding my preferences...
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I honestly think it's less a question of which tool does what you want, and more a question of which tool fits your skill set and comfort level. both psake and rake+albacore have pretty much the same functionality. they both work very well. they both make your job very easy compared to other tools. but they have different approaches and mind sets, based on their background and where they come from.
are you more comfortable with a stack of tools that is all Microsoft platform? or are you comfortable with tools that are outside of the typical MS stack, but still provide value to people who are building software on the MS stack.
would you rather have support for your build tool right out of the box, with powershell? would you rather have a c#-like experience, with the ability to incorporate C# and other .NET code into your build process? do you want the ability to use the various plugins for powershell that help automate various aspects of windows, for deployments?
or would you rather have a cross-platform, very open system that gives you a more natural language experience with the code you write, in ruby / rake? are you comfortable with *nix style tools and commands, ssh, and remote process execution for your deployments?
you say you don't mind installing ruby on the build server, which answers part of this question and leaves you open to the options. there's a lot of questions still needing to be answered, though.
if you are more comfortable with the MS stack and want the out-of-the-box experience of an MS based tool, then go with powershell. if you want to expand your horizons and get out of the MS stack a little, with a tool set that is easy to integrate and work with, to get a taste of what the Ruby world is all about, then go with ruby/rake/albacore.
i hope that helps, even though I have not given you a direct answer on which one you should use. hopefully i've given you enough to think about to determine which one is better for you / your team, though.