raid5 or raid6 and the maximum size

hi,
i'd like to know the current state of the raid6 driver. we are now switching one of our server from 8 x 200GB to 8 x 300GB(or 400GB).
so which is the better if we use:
- 7 x 300GB in raid5 and one 300GB spare or
- 8 x 300GB in raid6?
it seems the same to me since both case we can get two disk faild and the same capacity. so why we choose raid6 over raid5 (when raid5 seems to be more stable then raid6)?
another question is there any upper limit for one raid5(6) partition? i read 1TB and 2TB too (although we already have a raid5 which is larger then 1TB). these are still real limit? or what is the current limit?
thank you for your help in advance.
yours.

-- 
  Levente                               "Si vis pacem para bellum!"