Which iPad 3 should I get?

There are three considerations: storage capacity, price, and 4G/3G connectivity.

Let's start with price. The iPad 3 prices will probably be very similar to, if not the same as, the iPad 2.  As a reminder here is the pricing matrix as presented by Steve Jobs last year.

Next is storage, which is a really hard question to answer since it's a specific to each person's uses. Roughly speaking for 85% of you 16 Gb is plenty, for 10% 32 Gb is plenty, and the last 5% carry around way too much data.  Just look at your iTunes to see how much storage you are currently using.  Then consider what exactly you'll be syncing to your iPad...

Personally, I would never put music on my iPad.  It's already on my phone which I always have with me. If for some reason I'm without my iPhone but I have my iPad (can't think of a single scenario for this, but I'll play along), then I would just use Pandora to stream music to my iPad. 

I would also never put pictures on my iPad.  I take all my pictures with my iPhone, so it's much easier to keep them there and just let iCloud sync the most recent 1000 to my iPad for large screen viewing if necessary.  And 1000 pictures takes up minimal to no space.

That leaves only apps, books, and movies to take up 16 Gb.  I don't keep a movie library on there, so that's plenty.  I'm getting the 16 Gb version.  

Finally, 4G/3G connectivity or not?  That's easy, unless you travel a lot or have a daily commute, stick with WiFi only.

Here's the typical use cases to help see where you fit.