Well, depends on how you play. For group pve when you get your magicka from other sources than recovery (like elemental drain, siphoning attacks, necrotic orbs) the magician is better. For high regen builds that you often find in pvp the arcanist is most likely better, but to be sure you have to calculate it for your character details.
if you have 167 CP then 100 for regen else for cost otherways put some into cost until next increment will be less then regen increment (0.1-0.2) and then - all to regen.
For most classes in PvE, you'll want to go 100 into Magician and 67 into Arcanist. This does not change much for different races. I've a spreadsheet to demonstrate this but it is not very straightforward and given the amount of confusion my spreadsheet on Mage CP has generated I'm not going to advertise about the Thief CP spreadsheet here. Also the 100 into Magician and 67 into Arcanist doesn't really change for most endgame PvE builds so there is not too much point in using it. But if you're interested you can click the link in my signature and find the appropriate section.
I have split these every way you can think of. If you are at the CP cap, it doesnt matter that much. I used to always go 100 into reduction and 67 into sustain. But recently I have opted to get 75 sustain (for the harvesting passive), 10 into tumbling, and the rest into reduction. I cant tell much of a difference in my magic sustain, but I can roll more and harvesting nodes is WAY better. If straight PvE, you prob dont need the tumbling.
Edit: To simplify the math, the faster the rate of spell casting, the more reduction is favored over regen. In PvE, when in combat, you cast as fast as possible. So reduction is generally favored. In PvP, there is more downtime so regen becomes more important. @Asayre is definitely the math guru around here. Makes my life a lot easier. haha