EDIT - The electician is the expert and he would tell or supply you what you need. The question should really be "mr electrician, do I need a 125 or 100A Sub Panel? but usually they just supply these for you.
He'll size it according to the what's connected to it and the number of circuits that are off it.
Since you asked about the differences between the sub-panels. I looked it up and here are the specs of each.
Comparing a 100A and 125A from Seimens.
- 1 Phase, 3 Wire, 120/240VAC
- 12 Spaces, expandable to 24 circuits maximum with the use of QO tandem breakers
- Type 1 Indoor enclosure
with automatic flush adjustment and door
- CSA approved for vertical,horizontal or inverted mounting
- Lifetime Warranty
- 12 Circuits expandable to 24 (using space-saver breakers)
- Certified for use with any combination of full or half module plug-in circuit breakers
- Certified for mounting vertical, horizontal or inverted
- Suitable for copper or aluminum conductors Rugged construction and reliable performance
-A complete line of accessories give Loadcentres the flexibility to meet application requirements
So it fits the same number of breakers actually. That suggests that inside the bus bars are a little bit heavier rated (larger) in the 125A version. Which means:
PRO's: More available power if you need it - e.g. a greater number of higher ampage breakers. Or a larger number of lower amp ones. e.g. a configuration might be 4 x 25A breakers + a 15A and 2 x 5A. - 7 breakers but maxing out the capacity. You couldn't do this in the 100A version.
CON's: Appears to cost more.
I would suggest unless you can see that you've got a massive load, 100A will be fine.
If you're using your subpanel for a home workshop and you have several high current power tools like a table saw, air compressor, welder etc... then go with a 125A and run lots of circuits for the workshop. Otherwise, you may just be spending money for no real benefit.
Actually - 125A might be too small for most welders but I'm not familiar enough with the supply power they need so if you're going to run a welder - get that checked out with your electrician.
Ask your electrician what's needed if you'll be the one supplying the parts.