For WP it is. WP7.8 will get the full WP8 "experience" but no WP8 apps. I even imagine the experience will be better on WP7.8. There's lots of things CE isn't, but it is light weight and easy going and it is a RTOS. NT is a monster in comparison, and at least on any X86 Windows pre 8, it has no realtime capabilities at all, the timing accuracy is non existent.

I don't know if Win RT, the "RT" stands for RealTime ? In any case I can't believe they just smacked a vanilla NT on a phone without some heavy modifications to cut off some weight and include some realtime capabilities. But will it be more fluent than WP7.X ? I doubt it. There is also the power management to consider where CE is top notch, which cannot be said about NT. WP8 may run fluent and nice - for two hours.