In my opinion both of them are good enough
but if you ask me, i will go with sony xperia arc.
Reasons : Sony Experia arc s is cheaper than nokia 800. With the ceaper price you can get many better features than lumia 800 , like memory card slot(which nokia lumia has not) , 480 x 854 pixels, 4.2 inch(nokia lumia 480 x 800 pixels, 3.7 inches) and many more
nokia lumia 800 also has some interested features too ,like : 8mp camera Carl Zeiss optics, autofocus( sony arc s has not carl zeiss optict), 16 GB storage , high speed internet browsing, many new apps, smaller design than sony acr s.
I think those things can make you fell which one is better
but come back to my first post, both are good enough, it just depends with your choose
which one are you interested