We started Robomodo with Max in mind but moved to XSI for a number of reasons. A bunch of us used XSI while at NuFX and EA Chicago, and really liked the non-destructive modeling workflow. I am used to the snapping in XSI and have no problems with that at all, and prefer it to Max's, although it's been many years since I actually used Max in a full production.
Max's DirectX window is more stable and user friendly. XSI's directx window is kinda lame, we use the OpenGL window for realtime preview instead. With good NVidia cards and NVidia CG Toolkit 2.0 installed, we are able to do a good amount of preview (not all) in the OGL viewport.
Yes, XSI has no inherent tie to real world units, which is lame. our character guys work in centimeters and our env guys work in meters. we scale appropriate data in our pipelines to compensate.
We use Max only so we can continue to use ShaderFX. We are considering writing our own shader authoring tool in Ice when we're able to convert fully to 7.0.
As far as pelting, there is a plugin called "Roadkill" for XSI that is decent... but I still like a stand alone app called "UV Layout" by Headus (Phil Dench from Australia who also developed CySlice).
As far as tools deployment, XSI is hands down the winner in my opinion... we are able to deploy our tools updates seemlessly... and the scripting languages are very friendly to our engineering staff. XSI also has full collada integration which we're leveraging and adding custom data to.
XSI also bakes the best Normal Maps internally IMO, and also has the GATOR tool, which has probably bailed me out of over 1000 pickles. For those that don't know, Gator is a way to easily copy any type of vertex data (vert colors, uvs, shape animation, weights, etc.) across similar meshes of different topologies.
We see XSI as the right tool to grow a company around, their support staff is friggin rediculous, we ask them specific questions, send them scripts and test data, and have their engineers looking at our stuff sometimes same day. They send people to our office to work with us on site on our cornerstone issues as well.
They are smaller and more agile, and aren't afraid to inovate.
Fun for the whole family.