If you have a fully fledged x86 system without limitation, low power requirements or interfacing with embedded hardware as I seem to understand from your application then WinXP would win hands-down for me. WinXP has the horrible boot sequence and more venerable to security attacks but as this is not a hand-held consumable product I think I'd live without WinCE's finesse.
Your application, and the ad campaign in particular, are more important than many of the features of the OS, so I'd pick the most straightforward OS to develop on where the development OS is very similar to the target OS.
I would suggest a slight alternative to WinXP Embedded Standard. I would suggest "WinXP for Embedded Systems" (WinXP FES) which is marketed as "Windows Embedded Enterprise". This is the full version of WinXP (as you used to have on your desktop) fully supported to 2016!
The reason I'd suggest WinXP FES is for reduced upfront costs in development tools and development time, faster time to market and with the likely number of units the few extra dollars per system is not likely to be of an issue.
WinXP FES is not available through normal retail distribution (as WinXP has been discontinued), but is available though the embedded channel, the same place as you'd get WinCE and Windows XP Embedded Standard.
For those that do not know you'd need to fill out a declaration that the embedded device will not be used as a normal desktop computer doing office type functions. As long as you can demonstrate that the embedded device is not packaged as such then you'll typically be allowed to license WinXP FES.
Follow the registry hacks in this presentation to set the boot logo image, booting without the "Desktop" into your application (Kiosk Mode), suppressing pop-up messages etc. to make WinXP look more like an embedded OS.