The comparison for the old stat blocks versus the new ones is best done by looking at versions in the Monsters of the Multiverse compared to their old versions.
From spot checking they seem to be the same CR, and look to me to be roughly equivalent in offense, with the new stat blocks having about twice the hp of the old wizard caster ones and fewer possible magical defenses.
The Evoker for example is CR 9 for both versions with a constant 15 AC from mage armor.
In the old version it is a 12th level caster with 1 slot for chain lightning/wall of ice, and a bunch of slots for cones of cold and counterspell, fireball, lightning bolt, and misty step. It has 66 hp.
The new version has 123 hp and no counterspell as its defensive changes (also no misty step or stoneskin).
Offensively it has an at will three shot multiattack ranged attack arcane blast that is not a spell for 25 hp each. It also has a usually every other round (recharge 4-6) scultped explosion for 9d8 not spell damage of choice of energy and knocked prone to a 20 foot radius sphere that carves out three friendlies, save for half. It also gets one wall of ice and two lightning bolts per day as decent combat spell options.
For the old one you need to know core spells and how to use slots. For the new one you need to figure out the new powers. The new one is generally simpler to manage on a resource management track.
If I was running a 12th level party and having them face a BBEG with 3 hench mages for artillery I would probably want to use the new version over the older one.
If I was using an evoker as the BBEG for a level 6 party I might prefer the older statblock for more dynamic options.