I noticed that most Square D sub panels are 100 AMPS, but Siemens has one that is 125 AMPs (maybe other brands also, I haven't checked).
Is there any differences I need to know about when I suggest a 100 AMP sub panel versus a 125 AMP (or bigger) sub panel to my electrician? 

In this case the more AMPS the less wire he will have to run.

I am thinking about discussing this with him as an alternative to make it easier on him when he rewires my house, since the main panel is almost 50 feet away from the part of the house that needs 80% of the rewiring.