I bought an S360, and I decided to stick with an extra regular battery. I don't think the extra size, weight, and price of the extended battery is worth it. Here's why:
The capacity rating of a normal battery is 4800mAh. The capacity rating of an extended battery is 7200mAh. That's an increase of 50% capacity.
The normal battery has been shown to last around 4 hours in practice. An extended battery should last you around 6 hours. So the scenarios you are looking at are:
8 hours of battery life (2x regular batteries) or
10 hours of battery life (1x regular battery, 1x extended battery).
It does not seem like it is worth an extra $100 (on top of an already expensive battery), plus the added weight and size, to get 2 extra hours of battery life from 8 hours to 10 hours. I can't think of a single time when I intended to use my laptop for more than 8 hours away from an outlet. You can check in at JFK, sit in the waiting lounge for 2 hours, fly to London, and still have a little battery juice left.
So, that's why I say, normal battery all the way.