I'm jumping in pretty late here, and you may have already decided what to go for, but here's my take:
Definitely Win CE based. Whether you choose Win CE or Pocket PC as your OS is really more a matter of graphical display preference IMHO. I don't have any personal experience with Palm whatsoever, but I understand that they haven't really been commited to backward compatibility when releasing a new version of their OS. You'll never run into that problem with MS, and I think that's a very important point when chosing an OS.
I also understand that Win CE has surpassed Palm in sales some time ago. You definitely want to to acquire skills for the growing market. Also, learning Win CE based programming may open up new venues such as embedded development.
Go for .Net as your development platform, unless you're already well versed in windows programming in MS Visual C++. While the latter can be downloaded for free from MS, it takes significantly more effort to develop in C++ than in .Net. Unless you need the raw power that only a well written C/C++ app can provide, .Net provides the best tradeoff between execution speed and development effort IMHO. The license cost of MS Visual Studio .Net is easily offset against a significantly shorter time of development in most cases. MS is/has pulled the plug on eMbedded Visual Basic, btw, since you can get the same ease of programming in VB .Net, w/o eMbedded VBs severe limitations. Whether you choose to write in C# or VB .Net is mostly a matter of personal preference, BTW.
Any touch screen device can do basic signature capture. Check the web for 3rd party controls that you can use in your app.
http://www.componentsource.com/Default.asp?bhcp=1 would be one place to look. OTOH, the signature capture devices used in, e.g., banks, have more advanced features than what you can get off your PDA, such as pressure sensing.