I'm not sure what a "last" is but apparently Sidi's are made on a 15 degree last. At least that's what a Sidi rep told me a long time ago.
I used to ride Diadoras but switched to Sidis. I have flat feet and the Sidis seemed to work better for me - I'm on my 5th set or so, and I keep each one for a few years. They fit me really well. I try everything that comes out - Time, Carnac, Specialized, Northwave, Diadora, etc. But I inevitablely return to the Sidis.
The one disadvantage of the normal sole was one I never realized until I saw one pro chiding another for using the "flexi-flyer" shoes at a team presentation (Sidi did not sponsor the team). The non-carbon Sidis are a lot more flexible than many of the new carbon soled shoes as the chiding pro showed his well-known teammate (and me, in the background). The other pro shrugged and said something about racing in "aero shoe covers" the whole season to cover up his non-sponsor shoe choice. To his credit he decided to use the sponsor's pedals which everyone there apparently hated.
However, the newest Sidis (all buckles and string ones) seem to have lower heel cups which I dislike. My achilles feels unsupported and it feels like the show is about to fall off my foot. Since I never had problems with my achilles, I didn't want to change the way they were supported (or not). So I have to stay with the "lessor" strap and buckle Sidis. I recently got the one with all straps as well since they also have a deep heel cup (they are still new, they are my spares now).
Diadoras, for me, were too wide and the toe box too square. However I desperately wanted to use them (not sure why - price? weight? stiffness?) and tried to get along with them. Eventually I gave up. Nothing wrong with them, just didn't fit as well.
An interesting thing - if you line up at a bike race around here, most of the racers will have on Sidis. With so many other shoes out there, and so many of them cheaper, I think it's telling that more racers use Sidis.
cdr