I know that this may be late, but I think I have an answer for you:
MXM and Discrete GPUs are equal in power, but MXM can be upgraded while descrete GPUs are soldered to the laptop itself.
I'm sure you know this already. But here are the advantages and disadvantages:
MXM:
+ Upgradability
+ Lack of the need to buy a whole new laptop
+ Modularity
- MXM chips are quite rare to find/lack of availability
- MXM slots will be out of date very quickly (Meaning you can only upgrade once or twice)
- Expensive technology
Discrete:
+ Compact form
+ Cheaper to manufacture/mass produce
- A whole new laptop has to be bought if the GPU does not meet today's standards
- Lack of repairability if it dies
At the end, it depends on your personal uses.
If you need a laptop that needs to be upgraded VERY constantly to achieve max graphics, go for MXM.(Note: Being a hardcore graphics enthusiast will help you in here. But as a hardcore gamer, it won't help you that much.
If you are a mainstream gamer, go for Discrete. If you are a hardcore gamer... Just go for discrete. It's cheaper and has advantages that outweigh it's disadvantages.