Hey guys, this is me getting out of my cave and sharing a bit of opnion witht the rest of the world
I work with XSI most of the time, alone with LW, Modo, Messiah, Maya and even houdini, what ever it takes to get the job done. Yeah PointOven is a readGreat helper, so is FBX and collada 
So why would anyone want messiah along with XSI! it does sound redundent sure, but sometimes doing stuff in XSI is just a killer. one of the main problems in XSI is stability. Somewhere along the tranisition from SI3D to XSI something must have fell from the code, because XSI is not stable at all, and what makes it worse is that during the transition periode and while XSI was first a joke (ver 1.0 up till 2.1) big studios stopped relying much on XSI and switched mostly to maya, that caused allot of problems for SI since they stopped getting big studios bug reports and user feedback. if you compare the stability of SI3D and XSI the latter looses big time, it doesn’t even stand a chance infront of the complex huge stuff SI3D used to handle.
Now XSI offers alot of great features. over other packages of course, but it looses alot also. mainly what makes XSI great is the workflow. it has the best workflow ever to get things done FAST. not on the long run though or for complex scenarios. SI people thought about everything for the workflow, it’s just fluid. for an example, Constraints. It’s so easy to implement, rearranged, edited, muted, shared…etc, but it’s limited when it comes to layering them togather. and hitting a cyclic check is always the first thing you get before even trying your system. Thus the Modeling, Shape animation, Animation and secondary modes is a great helper, and IT DID HELP alot in solving alot of the bumps we used to come across, it’s still has it’s faults. now if you want to compare the fast use of such techniques with messiah then yes messiah will loose.
so what makes messiah a choice. simple! Speed. Messiah is faster than XSI, and more stable I’m afraid. now for cartonney characters or not? well that depends on how much your going to persue the application and demand raw power from it. So what speed I’m talking about it here? it’s how fast the application interacts with you, the speed of the viewport, openGl…etc. Messiah is simply faster, thus it gives you more time to animate and tweak that motion, and experimnent more than you do in XSI. Now don’t get me wrong, there are many ways to go around this in XSI, you can revert to layering, digital assets, proxies, resolution characters. it all depends on how much slow is your scene plus how much you need to see while animating. Buttom line is, if you want to just animate and feel free, messiah is worth it, especially if you want to express your self more with complex character and don’t want to do a flipbook every couple of moves 
So what about stability in XSI. well this is one of the things we ran into alot almost in any production, we think we nailed that pitfall and learned ower lessons but it keeps coming up again and again. For instance, the AutoRigger. well that’s one fast way to rig an advanced rig in no time, then it comes the animation stage. the more you tweak your character, the more it breaks. one of the areas that your sure to flip on you and break everything for you is the shoulders. now the autorigger comes with Synoptic view, like Armetures. it’s nice and great. but don’t freezeModel your object, or change hirarachy to it, because it will simply break and you wont have that window again. stipping down information and making a clip out of it is also a big hassle and very good technical skills to find and solve the problem, most of the time it works with a workaround. Scripts are also nice, and Scripted Operators is even better, untill you handle your character to an animator and he starts screaming when they break up, it happens, they might click something that irritates the Scripts and there you go, start debugging. I can go on and on for hours here, the buttom line is, every software has it’s pitfalls and XSI has it’s good share of them. so the other buttom line, yes messiah is a good plus if you want to animate and not worry about that kind of technical problems and trouble shooting.
quiet honestly both programs are good, and yes Messiah looses alot of features commpares to messiah, but even XSI ess. unless you have the advanced version, where you get Syflex. Hair and Behavior. but belive me guys, Syflex have more problems than you can imagine. sometimes I forget it’s a bless and I start to think it’s a mess. the Maya version is alot more faster. stable and reliable and it get things done faster in the end. as for Cloth, it’s nice
JoeAlter is happy, and you get hair. on time I was doing hair for a character, and I spent alot of time fixing it, building the effect system. praying it wont crack and the hair jumps. once I was done, saved it as a model and loaded it in the main scene. wait a minute why the hair is 10 meters away from the character? well the problem was with XSI 5.0 and if you don’t have a maintance contract like me I had to wait for XSI 5.1 to solve it. since it was a bug. then again when XSI 5.1 came out, all my renders had black holes in them and I had to change the lighting, remove FG or Area lights since they are a big cause of those, untill they released XSI 5.11 which they said it solved it, but you had to recomplie all your shaders to the new ones, of course that was a lie beacuse the problems stayed there and of the community memebers traced the problem to an antialiasing issue with mentalRay and wrote a lens shader to fix it, not SI folks 
Anyway enough about me and my problems, I’ll stop nagging now and crowl back to my cave and trouble shoot some more
Hope this wasn’t a long meaningless post and people actually gained something from it.
Cheers
I3D