maya.cmds VS pyMel.core
Tl;DR
Just use PyMel(modern python in maya). When something won't work, use CMDS, and if you can't find it or get it working in CMDS(classic maya python), use MEL(old school maya scripting). This is pretty much the norm, lately. More experienced/older users tend to use CMDS and OpenMaya for some good reasons that you'll learn about later on(not important for starting off). There are reasons not to use PyMel sometimes, but you'll figure that out faster through trial and error(and reading docs) than trying to understand it before you've started programming.
Longer Explanation
It's an architecture preference, mostly. Would you rather write in an object oriented or functional way? That example is typescript, but the ideas are the same. OOP is considered more organized because it enforces an organizational structure, it's usually easier to read, and a lot of people find it easier to learn. People lean toward OOP for front-end (user interface) stuff, so web people lean heavily toward OOP.
CMDS is closer to the MEL commands that are core and original to Maya. Most menu commands run MEL, just because of legacy. Maybe that's changed in the latest Maya. This is an overview of maya's language architecture:
(from Maya Python for Games and Film, one of the classic references, now)
Python has some features that are well geared for object oriented programming(OOP), so people that like writing in a "pythonic" style prefer PyMel because it has a PyNode object that has a bunch of methods. Some people worry PyMel is too slow. Imho the speed thing is more a question of functional vs OOP, it's easier to write fast procedures in a functional way than OOP. But a lot of people find OOP much easier to read, and a lot of the time "slow" is still 100x faster than some artist doing the same by hand.
