Simple question (maybe):

Have an existing character and would like to enhance the Magicka pool...

Is it better to enchant items with Fortify Intelligence or Fortify Magicka?

Both of those spells (Fortify Magicka and Fortify Attribute) have the same base cost of 1.0. But I feel like UESP isn't offering all the useful information; it's just a guess but I think that a Fortify Magicka 50 will cost less magicka than a Fortify Intelligence 50 for the same duration.

So, we can go down two avenues of thought: 1) assume that they cost the same (1:1 ratio) or 2) assume that Fortify Attribute costs more than Fortify Magicka (would require testing).

1) If they cost the same, then you're always going to get the bigger bang for your buck boosting your intelligence. It governs multiple skills in addition to your magicka pool. Plus, remember that 1 unit of intelligence = 1 unit of magicka (so if you have 75 Intelligence, you'll have 75 magicka). Unless you have a magicka multiplier greater than 1.0 (the default). Which is likely if you're playing a Mage-type character - and in that case, you're REALLY going to be better off going with Fortify Intelligence.

2) If we assume the second... then it would require testing, but your basic formula would be this (assuming Spell Values of 50):

(Magicka Multiplier)*(Fortify Intelligence 50) = Magicka Boost-FI

vs.

(Fortify Magicka 50) = Magicka Boost-FM

Compare the answers using this formula (costs for Spell Value of 50):

(Magicka Boost-FI) / (Spell Cost-FI) == (Magicka Boost-FM) / (Spell Cost-FM)

If the left side is greater than the right side, than you should stick with Fortify Intelligence, if not, then stick with Fortify Magicka.

I was thinking of CE enchantments on items... so the cost, though substantial, is a one time thing, so I'm not really concerned about that...

Given that I don't have a multiplier on this character, 1 INT = 1 Magicka, but it's fair to say there are additional benefits to fortification of INT, so I'll stick with that.

Direct fortification of Magicka never really occurred to me until I was awarded the Belt of the Hortator....

Even with CE, the spell cost will still have a large effect on what you can do. So, just pay attention.