If you are talking about jewelry enchants, then typically reduce cost is better, but it can depend on a few things.
Generally gold 160 cost reduction glyph reduces spell cost by 203, and spells can be cast about once per second. So it effectively nets 203 mag/s. Recovery on the other hand adds 169 recovery, which ticks every 2s. Effectively a recovery glyph nets 84.5 magicka/s.
Reduce cost gets weaker if you are stacking spell cost reduction % bonuses. Recovery gets stronger as if you are stacking magicka recovery % bonuses.
Cost reduction also does nothing when not casting a magicka skill. This could be: using a heavy attack, casting a stamina skill, moving between fights, doing mechanics, dodge rolling, fights where the boss becomes invincible temporarily, etc.
There is no overall better choice. It depends on your gear, how many cp you have, the skills you use and many other things.
As a general rule you can keep in mind, that cost reduction becomes better, the more magicka you get back from different sources than recovery (e.g. heavy attacks, magicka steal, class skills, ...) and recovery becomes better when you have a lot of percentage based multipliers for it (e.g. altmer race, light armour, multiple pvp support skills slotted, ...)