If you are playing a mono colored deck with few utility lands Lens will be better. Doubling Cube is better the more utility lands and non-land mana sources you have. In a Neheb build you would generally want doubling cube if you want the extra mana effect. Really you'd want both though.
I would say it's the cube, but it really depends on your build. If you have a few XR spell which can use all the mana then the cube is certainly nice but somehow needs Neheb, the Eternal out to function. Let's make some calculations:
1) Assume you have n mountains out and you already imprinted a mountain on the lens during the last turn. Then you get 2n mana with the lens. With the cube you get 2(n-3). In that sense, n needs to be bigger than 4, otherwise the cube doesn't yield anything.
2) Assume you have m mountains, n utility lands and l mana from Neheb. For the lens (still assuming that a mountain is imprinted) you still get 2m mana. But for the cube you get 2(m+n+l-3). So the difference between those is 2n+2l-6. So if n+l>3 then you're good to go! I would suppose, with Neheb out, this is often then case.
Let's say you play 7 utility lands, which is not a very high number even in mono color decks, and 38 lands in total then the probability of drawing one such land in the first 5 turns is around 60,7%. So more than every second game you should get a utility land in the first 5 of your turns (without drawing additional cards!). Playing some rocks in addition you should have Neheb out on turn 4 or 5, so on turn 5 or 6 you can get a value for l which is certainly higher than 2, which makes the cube better.
If you can, just run both. When I built Endrek Sahr, Master Breeder, I threw in every available mana multiplier, because I want to see at least one a game.
I would actually argue against simply running both. I depends a lot on the composition of your land base. So let's assume you run 12 utility lands, e.g. Ancient Tomb, Nykthos, Shrine to Nyx, some that grant haste etc. You will easily have 10 to 14 utility lands. Now assume you run 38 lands in total. So what's the probability of drawing at least 2 utility lands until turn 5 without extra draw effects?
for 10 utility lands: 34.8%
for 11 utility lands: 39,7%
for 12 utility lands: 44,5%
for 13 utility lands: 49,1%
for 14 utility lands: 53,5%
I think if you run 12 utility lands the probability is rather high of drawing such a land and thus making Extraplanar Lens rather bad - you still need to exile an existing land on the field! I guess there are better options in that case that might suit the deck better, e.g. more control elements and interaction. But then again, it depends on the meta - if you run into a lot of artifact hate you should forget about the lens. It sets you back too much. Otherwise, sure, you can test it out and see how it suits your playstyle.
Don't forget that you have to atleast have 8 mana to go plus with the cube. With the lens you usually get up to that mana before the cube. But Lens does cost a land when it etbs. SOmetimes it just won't give you value since its a big target.
Don't forget that you have to atleast have 8 mana to go plus with the cube.
7, actually. 3 to activate puts you down to 4, doubled to 8, which is greater than 7.
The main issue you have to consider with Doubling Cube is that it pushes you towards dumping a whole bunch of mana into spells and abilities during a single step or phase. With a normal mana doubler like Extraplanar Lens, you can spend 2 mana here, 4 mana there, and all is well.
Card advantage is not the same thing as card draw. Something for 2B cannot be strictly worse than something for BBB or 3BB. If you're taking out Swords to Plowshares for Plummet, you're a fool. Stop doing these things!
I would say it's the cube, but it really depends on your build. If you have a few XR spell which can use all the mana then the cube is certainly nice but somehow needs Neheb, the Eternal out to function. Let's make some calculations:
1) Assume you have n mountains out and you already imprinted a mountain on the lens during the last turn. Then you get 2n mana with the lens. With the cube you get 2(n-3). In that sense, n needs to be bigger than 4, otherwise the cube doesn't yield anything.
2) Assume you have m mountains, n utility lands and l mana from Neheb. For the lens (still assuming that a mountain is imprinted) you still get 2m mana. But for the cube you get 2(m+n+l-3). So the difference between those is 2n+2l-6. So if n+l>3 then you're good to go! I would suppose, with Neheb out, this is often then case.
Let's say you play 7 utility lands, which is not a very high number even in mono color decks, and 38 lands in total then the probability of drawing one such land in the first 5 turns is around 60,7%. So more than every second game you should get a utility land in the first 5 of your turns (without drawing additional cards!). Playing some rocks in addition you should have Neheb out on turn 4 or 5, so on turn 5 or 6 you can get a value for l which is certainly higher than 2, which makes the cube better.
So, in conclusion, I'd say stay with the cube.
for 10 utility lands: 34.8%
for 11 utility lands: 39,7%
for 12 utility lands: 44,5%
for 13 utility lands: 49,1%
for 14 utility lands: 53,5%
I think if you run 12 utility lands the probability is rather high of drawing such a land and thus making Extraplanar Lens rather bad - you still need to exile an existing land on the field! I guess there are better options in that case that might suit the deck better, e.g. more control elements and interaction. But then again, it depends on the meta - if you run into a lot of artifact hate you should forget about the lens. It sets you back too much. Otherwise, sure, you can test it out and see how it suits your playstyle.
Who needs Colours?
My most played EDH deck:
X Kozilek, the Great Distortion
UBR Nekusar, the Mindrazer
The main issue you have to consider with Doubling Cube is that it pushes you towards dumping a whole bunch of mana into spells and abilities during a single step or phase. With a normal mana doubler like Extraplanar Lens, you can spend 2 mana here, 4 mana there, and all is well.
Two Score, Minus Two or: A Stargate Tail
(Image by totallynotabrony)
On phasing: