Are you talking about using either 2 cm or 3 cm slabs? I've never actually seen them measured in inches.

Presuming that's what you mean, there are a lot of threads on that topic here. Type "2 cm vs 3 cm granite site:gardenweb.com" into your search engine and you'll turn some up.

The bottom line is that you can use either for your islands. The 2 cm slabs are placed with a plywood underlayment and generally have another 2 cm strip of granite laminated onto the edge to give a nice edge finish and cover up the plywood. 3 cm slabs don't need an underlayment and have a nice fat edge for finishing built in. 2 cm slabs are more popular (i.e. more available) on the west coast and 3 cm more popular on the east coast, but that is not a universal comment on availability. I live on the west coast and I have 3 cm granite. Functionally you can go either way and be comfortable about your decision.

When you're looking at costs, be sure to factor in the extra fabrication expense for the laminated edge and the extra expense of fabricating a stone that eats up the fabricator's saw blades like the really hard quartzites.

2cm actually can end up a lot more expensive than 3cm. There's more labor (and risk) in fabricating it because it's not as strong and needs that subtop. And if you get into a mitered laminated edge, it's for sure more expensive than 3cm. Be sure you understand the implications of how your edge will look with the buildup. With a stone with movement, that can be challenging even to an expert fabricator.

I used the same material in my kitchen (3 cm) and bathroom (2 cm with mitered laminated edge) and the fabrication of the latter was less expensive on a psf basis than the former.

Thank you for your great responses!
The use of a plywood subtop would be very problematic on inside sink edges, as well as some areas that might be seen from below.
I guess 3cm it is.
Maybe not that much more expensive...

The plywood won't show at the sink. They cut it back from the sink so it is not visible.

I've heard it's regional too. In Southern California, there's not a lot of 3cm around. Back east, I understand it's the norm. So it could also depend on where you're located as to what is available and what it will cost. Bucking the norm for your area will probably cost more (or severely limit your options.)

We live in Connecticut.
3cm seems to be the local standard for kitchen countertops.

I just spoke to a stone supplier in Van Nuys and he assures me that a majority in California is 2cm and I should have no trouble...

Probably go with 3cm because it's the look that people expect around here.
But the cost is crazy for this stone.

It's Azul Macaubas, Martin. Look at this gorgeous thing!

When you pick the exotic beauties, you pay for them. Listen to this description from Marble of the World:

Azul Macauba is a highly sought after exotic and exclusive quartzite. Its striking intense blue colors on a white grey background accented with white, brings an air of elegance and sophistication to any project.

It's not like you're considering Ubatuba or Giallo Ornamental or others that are widely available. You'll pay for the exclusivity, the beauty, and the hard-to-cutness. I'm jealous.

MartinJW:

According to the MIA, you don't need a subtop on 2cm stones that are sound. If you like the look of the thinner edge, go for it, no mitering or build-up is required:

Here is a link that might be useful: Page 3