My question is, why not both? I don't think I have ever seen a build that makes use of the Hybrid Glitch.

Some of the different Hybrid-versions have major drawbacks like that the game still thinks that you're a vampire lord, which makes everyone hunting you, or that the weakness to sunlight still applies. At the same time it makes you unable to enter castle Volkihar, without beeing attacked by the vampires that think you're a werewolf, or Fort Dawnguard because they think you're in one of the beast formes.
Aside that, you have to do some missions in the vampire & companions questline to get there, for some forms even the whole Dawnguard Questline.

I once has that happen to me by accident - I was a werewolf fighting a vampire and hag that had invaded whiterun, and I drank a potion of weakness to disease and some benedicial effect. I was pretty fun, save that feeding as a ww didn't count as vampiric feeding, and I wasn't a vampire lord.

25% Regenerate Magicka doesn't even come close to what the Ring of the Erudite does, and you have to track down a moon sugar for every 12 minutes of the effect.

I'm not gonna lie. I personally think the hybrid glitch is rather convoluted. Power gaming gone too far.

The Ring of the Erudite is by far the most powerful magicka regen effect in the game as it adds 2 base magicka regen. Your standard is 3, multiplied by 2 (100% base magicka regen) = 6. With the Ring of the Erudite it's 3+2 x2 = 10 magicka regen out of combat. Now add in the apprentice stone or Adept robes and you have 20 magicka regen per second. In combat it's only 1/3, so 3.3, 6.6 respectively.

25% would be 6 + (6 x 0.25) = 7.5 out of combat. In combat it's 1/3, so 2.5.

What's the multiplied by 2 from?

There are two values:
The base magicka regen, flat 3.0 magicka per second, is increased by the standard magicka regen, fortify magicka regen 100% (basically a x2 multiplier).

=> The flat 3.0 magicka per second gets 100% more magnitude (3.0 x 2) = 6.0 magicka regen per second.

The Ring of the Erudite adds 2.0 to the flat magicka regen
=> (3.0+2.0) x 2 = 10 magicka per second (out of combat).

Beeing in combat reduces the base magicka regen to 1/3 => 2.5 without, and 3.3 with the Ring.

If done right and with limits it could be something interesting. Just the RP for such a creature.... not to mention the nasty left field lore your would have to come up with.

Cheese and Rice, Relycs, that's a shit ton of math. I mean I took Calculus in college way back in the dark ages, but sheesh, it's a video game. Last time I checked I played those for fun, not for math, but to each his own. I can see how people would find that appealing. Probably leads to a far more efficient game, which I sort of suck at.