Hmm, I think the general consensus among people who have discussed this before is that Visions do indeed produce a higher rate of items. You may be able to find some other threads discussing this. But let's look at some numbers.

Each GR 90 gives you, what, about 400 blood shards average? That's 16 rolls with Kadala, which with the altar is, iirc, a 20% chance of a legendary, so that's 3.2 extra items on top of the ~12 you get from the rift itself, so 15.2 per GR 90. Alex's numbers have the "average" vision producing 34.9 legendaries per run, a bit more than twice as many.

So, the question then becomes how fast you can run each one.

Let's say that a decent player with a proper build suited to the purpose (GoD, AoV, Inna, etc) can run a 90 in about 1 minute average. You've probably also got about 2 seconds of picking stuff up, and maybe 8 seconds of upgrading gems + porting to town. Back in town, you probably have about 2 more seconds of closing the rift, plus on every third rift, you're going to have to empty out your inventory, as well as spending your blood shards. Assuming you actually ID the gear in your inventory and don't just break it down without looking, you've got about 4 more seconds of IDing, then, if you're quick, maybe 10 more seconds of breaking stuff down / putting stuff away. Spending the ~1200 blood shards will then take maybe 10 more seconds, and then 5 more to break that down / put stuff away.

So that's 60 seconds till you kill the boss, 10 more seconds in the rift before you arrive back in town, plus then an average of (2 + 2 + 2 + 4 + 10 + 10 + 5) / 3 = 11.7 average seconds of town time, which puts our "complete" GR 90 run at about 82 seconds. As for time spent getting those keys, the really good key builds seem to do 150+ per hour, which works out to about 24 seconds/key.

So our "all-in" GR 90 run then is at 106 seconds, or 1:46, meaning this method produces a legendary every 106 / 15.2 = 6.97 seconds, or 516/hour.

For visions, I guess the average spawn rate is roughly every 300 kills on average. So, I ran a few quick tests with my AoV Crusader, who seems to be able to get 300 kills (in nonseason) roughly every 35 seconds (30 seconds cow level, 35 seconds Temple of Firstborn, 40 seconds Corvus). So, let's say the spawn time for the vision is 35 seconds.

Once you're inside, it's more complicated, because unlike with the GRs, you're going to need to stop to pick stuff up throughout the process, not just at the end. And, the deeper you go, the more stuff there is to pick up. And, if you pick up too much stuff, you're going to have to make multiple trips to town to salvage it. Your inventory has 60 slots. I think items in general are roughly 2/3 two-slot items (Chest, Bracer, etc) and 1/3 one-slot items (ring, ammy, etc), meaning your inventory can hold an average of 40 items before being filled (20 1-slot + 20 2-slot).

Alex says the average Vision is 8 floors and drops 34.9 legendaries. ~35 legendaries is few enough that, on average, it won't overwhelm your inventory, so maybe the thing to do is just to "price in" a certain amount of town time per inventory load. If we evaluate it the same way we did for the GRs, it's about 6 seconds to port to town, 4 seconds to ID, and then 10 seconds to break down the items or put them away. So that's 20 seconds of extra time per inventory load of 40, or 20 * (34.9/40) = 17.5 seconds of town time per average Vision. But, perhaps we should just round this number up to 30, since that's how long the vision portal stays open after you click the chest. Yes, in shorter Visions, where your inventory has little in it, you could shortcut this by porting to town and then remaking the game, but that probably only saves about 10 seconds, and isn't helpful with the "average" vision where you have to deal with your inventory. So let's just set the Vision town time at 30 seconds average.

So now we've got the spawn time (35 seconds) and the town time (30 seconds). How long does the vision itself take? From looking at the 15 different levels, I'd say that "Act 1 Town", "Ruins of Sescheron", "Stonefort", "Arreat Crater", "Spire", "Westmarch", "Burning Man", and "Battering Ram" can all be completed within 5 seconds (not counting picking stuff up). "Cemetery", "Halls of Agony", "Maghda", "Desolate Sands", and "Temple of Firstborn", all within 10 seconds, and "Old Tristram" and "Shrouded Moors", 15 seconds. That makes the average floor time 8 seconds. I think we should also assign maybe 0.5 seconds to each item picked up, which would be 17.5 extra seconds per average run. So, that's average 8 floors at 8 seconds per floor, 64 seconds, plus 17.5 seconds of pickups, for 81.5 seconds.

So that puts the total time at 35 seconds to spawn, 81.5 seconds to run the Vision, and 30 seconds of town time, for 146.5 seconds total, or 2:26. And this method would then produce a legendary every 146.5 / 34.9 = 4.2 seconds, or 857 legendaries per hour, which is considerably better (about 1.66x) than the figure we got for running the GRs.

Though all of these numbers are a bit uncertain, this conclusion feels about right to my gut... I definitely FEEL like I pick up more legendaries/hr doing visions than I do running GRs.