Hi Faeroon
I am use both Xsi and Maya
Hereinafter is only reflect my opinion
Interface:
Xsi better than Maya
Node Capability:
Maya better than Maya
Operation Stack:
Xsi better than Maya
Polygon&SubD Model:
Xsi better than Maya
Polygon Disposal:
Maya is exceed
NURBS Model:
Maya better than Xsi
Multiple Surfaces:
Xsi some sort better
Deformations:
Xsi some sort better
Animating with Keys:
Xsi is equal to Maya
Constraints:
Xsi better than Maya
Linking or driven Parameters:
Xsi better than Maya
Expressions:
Maya is exceed
Character Animation:
Xsi some sort better
Shape Animation:
Xsi some sort better
Nonlinear Animation:
Xsi is exceed
Shader:
Xsi is equal to Maya
Texture Edit:
Xsi is equal to Maya
Render:
Xsi is exceed
Render Pass:
Xsi is exceed
Particle:
Maya is exceed
Cloth:
Maya some sort better
Soft Body:
Maya some sort better
Rigid Body:
Maya is exceed,Xsi is not support
Forces:
Maya is exceed
Fur and Hair:
Xsi better than Maya
Paint Effect:
Maya is exceed,Xsi is not support
Composite:
Xsi is exceed,Maya is not support
Hereinbefore is just a reference
I think use Xsi or Maya is not difference,the key is yourself
Several suggest:
If you want to create Character Animation,I think you should be use Xsi
If you want to create Particle Simulation and Specialties Effect,I think you should be use Maya
good luck 