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Brian, your skills, of course, will be helpful, but almost everything is different in XSI.

Nurbs modeling is not the strongest feature of XSI. It does not even support booleans on nurbs. You can achieve the same results as if you use booleans, but in a more hard way (by using projecting, trimming, constructing mesh from surfaces). And, of course, XSI does not import nurbs from other apps if they contain boolean operations.

In sub-d modeling you will miss some features from Hex and Carrara e.g. fast extrusion, automatic setting of working plane according to the camera position, displacement maps painting and textures painting. Actually it has some brush paintings, but they are weak as copmared to even Carrara and Hex (I don't even speak about ZBrush). However, XSI (unlike, say, modo) has almost all modeling features Hex and Carrara have. But XSI will give you many modeling features that not exist in Hex and Carrara, including local subdivision, weight maps (for making creases and spikes without adding new geometry), snapping options, and many, many others.

Despite XSI has no good texture painting tool, it has a lot of options in UV editing - more than Hex or Carrara have.

XSI is good in character modeling - rigging, weighting (called enveloping in XSI), animating. XSI has rich scripting features and operators stack (somebody at Polyloop called it "dynamic geometry on steroids"). It has no plant generation like Carrara (but there are 3rd party plug-ins).

XSI has top class render mental ray, and you should have to learn how to manipulate the render tree. mental ray is cool in realistic rendering and also have toon shader. Unlike Carrara, XSI Foundation has no hair, but more expensive editions of XSI do have both hair and cloth simulation.