At first it just came down to "do I want a sprinter's bike or an all-rounder's bike", but then I got hung up on the geometry. They're nearly identical except for the HT length. The problem is I'm really quite flexible and quite adamant about minimizing spacers, but at the same I don't know if the SST might position me too low for my stature.
Well... I'd actually say they are quite different in geometry.
The Stack and Reach figures tell the story. The SST is longer, in every size. (SST/ALT Reach = 380/363, 375/370, 386/370, 396/379, 405/389, 418/397 ) The Altamira is quite different there. However, the stack figures are very similar. (SST/ALT Stack = 513/517, 523/528, 535/540, 555/560, 574/580, 580/600) So, its not so much about getting the drop right as getting your preferred stem length and reach.
Also, the headtube angle and fork rake differ significantly as well. This effects trail and the handling of the bike. The SST is shallower in every size, and also has less fork rake... Both of which slow down steering and make the "sprinter" bike hold its line with more resistance to change. This also effects turn in, or "flickability" The SST needs a bit more of a heavy hand, but at 40+ mph, throwing the bike around, this is a good thing. Depends on what you're looking for.
Stack and reach is the way to go, as far as fitting these frames goes. And don't forget about the 15mm headset cone spacer, which bumps up the stack and decreases the reach a bit.