As i said for before..... the clarity of the videos will go down on the mobile youtube and you can never watch it full screen and you have stupid pop ups taking half of the screen. Do you want all that.
Regarding the copy and paste thing ... waht i do is open both uktube and youtube browser at the same time and long press on the youtube video after the search results come up..... select copy link location change window to uktube and click paste and show in widescreen .... wait for 4-5 second s and there video plays full screen. That is not much of a hard work. And on top of it it saves the clip for later play back.
Personally i prefer to use the large screen the n800 has to offer...