WinMo anytime. The customization options are AWWWEEESOMMMEE! Never been able to do that with my last 4 Symbian phones. Winmo is definitely recommended if you are a do-it-yourself type geek. Performance is great when tweaked properly even on a 201 MHz proccy on which Symbian sucks to boot! Some of the greatest Games are available for Winmo like Tomb Raider (full 3d, PC like experience), Call of Duty 2, Age of Empires (matches exactly to the PC version and very playable), Guitar hero, Virtual Pool 3D, Asphalt 3D (racing game, really nice), just to name a few. Apps are available for almost everything, you can ever imagine. There is even a command prompt and language compilers available for C#, VB.net, GCC (C and C++) so you can write code and compile it on device itself. I cant describe it fully here, it would take 10-15 pages! And this is with Wm 6.1, 6.5 is improving and 7 will revolutionize Winmo ecosystem, thats quiet sure.
Now, The two problems that i found with Winmo platform,
1. Since the phone audio is not routed through WinMo software interfaces, its impossible to record a two way conversation properly. The best software available is PMRecorder which can record other side voices but very low.
2. Camera is usually not very nice, better on high end devices, but doesn't matches Sony's cameras.
Thats it, apart from this, almost everything can be customized or replaced.