I would take Windows Mobile any day over Symbian. I found the latter to be slower and bland for my taste and also extremely limited possibility of customization and tweaks.... Windows Mobile 6 and later based devices are sleek and thanks to xda-devs, one can satisfy their inner g33k-urges everyday !!!
Windows 6.5 has already been ported to some present devices but still has a few problems.. also Android has been successfully ported to some Qualcomm MSM7xxx chipset based devices like Kaiser, Vogue, Diamond and some others... so buying any current-gen WinMo phones will automatically provide the capability to dual-boot 2 different OSes, and even at its present stage Android is rocking my Kaiser...
As far as the UI goes, you can do almost anything with Windows Mobile... what you have to do is provide some of your time at xda-devs and inner-urge to customize like hell.... if you have them, get a WinMo based phone and you are all set for one hell of an awesome mobile experience ...