Still they said all of the battlegrounds will either be 10, 15 or 25 players.
And that you cannot queue with more then 5 toons at once for a BG.
There won't be nearly as many premades, in rated battlegrounds as there are currently... as you can only be certain groups of five arrive in the same BG.

Given a rated battleground that rewards points which purchase the same rewards as rated arena...
I would assume the majority of the battlegrounders will choose the rated option.
Currently under 1500 rating, you get the same base amount of points, so there's nothing to lose and points to gain.
Which would mean, rated BG's won't be any harder then the current unrated BG's.

So the question remains, looking at the required win percentage to reach or maintain 'x' rating in arenas...
Which will be easier/harder to do - win that percentage while controlling 80/100% of your side but playing against five focused/coordinated toons, or controlling 20/33/50% of your side but playing against competition of much greater variation in skill and gearing?