Brodual and someone else (Shinji, maybe?) have good reviews comparing the two mods which can be found on IH's nexus page.
You need to install IH before you meet any horses, though, or those horses won't be 'recognised' by it and won't work with its system. EFF is great, thoguh, and I'm gonna try this combination on my next playthrough. Immersive Horses I haven't tried yet, you can assign horse ownership to your followers BUT they cannot ride them without EFF, according to the mod's description and reviews. Vilja, another follower mod, has her own horse named Bruse, who she can, I think, ride even if you do not own a horse. He will be most grateful for this wonderful gift. Inigo, a follower mod, can be given a horse if you speak to him while near a horse that you own. It's *meant* to be compatible with CH, but in my last game my EFF-handled followers never got their horsies (and I couldn't see any way to give them horsies through EFF) while Arissa (who is not and should not be handled by any follower mods) had her horse just fine (although she did bring it into Dead Man's Drink with her when dismissed.)
I've not tried that so IDK if you have to buy horses for them or if they just appear. Convenient Horses lets your followers (mostly) ride horses as soon as you get on yours, theirs will be 'summoned'.ĮFF apparently lets your followers ride without CH.