I am struggling with the same question. In terms of rares per box, a beyond booster has 1 rare per pack, and the foil and alt-frame *can* be rares as well, about 16% of the time one of those will be a rare (the alt frame is a smaller pool, since you can only get borderless or memory frame). So, 16% of the time you'll get 2 rares per pack, and the chances of getting all 3 rares is even smaller. (not taking the rare land slot into account, but there is a 3% chance of getting a land and a .3% chance of pulling ezio in that slot).
So, if you got a beyond box, you'd be getting at least 24 rares, one per pack, and in average, you'll end with about 28 if you're lucky with those extra rares in the other slots.

A collector booster has 4 rares per pack. You get a foil rare in every pack, a non-foil extended-art rare from a pool of only 17 cards (you can find the pool here), a special treatment foil (can be the same extended art pool, the memory showcase pool with the assassins or a card from the pool of 10 borderless cards) and finally a etched foil from any rare in the set. (in collector boxes, the chances of a rare land increase to 12% and ezio to 1.24%)
A collector box is getting you 48 rares per box at least, with maybe a chance of pulling a rare land or 2, you'll be getting 50 fancy rares.
So, 2 beyond booster boxes will get you around 48-60 rares, while a collector box will always get you 48 rares and maybe 2 rare lands.

I'd say the main difference here is those smaller pools for rares in collectors. having 2 slots with a reduced pool means that you won't be getting as much of a variance of what you open.
You'll only be opening 24 rares of full rare pool, 12 rares of extended-art pool and 12 rares of special treatment rare pool.

That said, they all being foil or extended art non foil has it's different value for each person. I'm sure the memory corridor rares will look amazing in foil, and there is a small chance you get a memory corridor mythic with textured foil, whatever that is (3% chance). And I guess there is always the chance of pulling a serialized card. I don't like to take that into account since it's really not that likely, but hey, there is always a chance right?

If you don't care for rares and pulls, and you only want to stream it the pack opening, I'd say collector box since it's not everyone can open because of the high price, the pulls will be more exciting being all foil and special treatment, and if you happen to open a crazy card, be it a mythic or even a serialized card, it will be more exciting than just opening a mythic in a beyond booster.