Vert beat me to a lot of the points I was going to mention (primarily about the non-viability of dual color builds). If the real question is, should you run black or red... well I think there is a good case to be made for black.
Mana:
- My esper build needs BB for Ob (who gets played late), and languish. If you ran radiant flames instead of languish... well that's debatable. Sure, you can play it with 2 colors, but languish can hit an extra toughness. I might give you a slight edge to radiant flames here, but it's very small.
- Manlands: I'll take lifegain anyday in this sort of deck. Esper wins here.
That's... really the only mana difference between UBw and URw. It's not a huge deal.
Removal:
- Ignoring sweepers (mentioned above) we're talking about anguished, push, grasp, murder vs red answers. I love fatal push, it's really not hard to trigger revolt between our EWs and clues from confirm. Sure, magma spray is good sometimes and fiery impulse is great... but red is all burn. By running black we get some situational removal (like push) and some hard removal like murder or even anguished unmaking. I have a hard time buying the argument that fiery impulse is enough of an improvement over push to justify losing anguished unmaking.
Finishers:
- Lili, ob, soren vs chandra and shahili. No contest for me here. While they can all finish a game, Lili comes out fast and provides reliable board control, while ob and soren provide card advantage and board control. This advantage part is key (at least to how I play). Absolutely no contest here for me, black carries it. (Yes yes, enigma drake is cool, but it's vulnerable to a lot of stuff).
Sideboard:
- This is also pretty onesided, dead weight, scarab feast, complete disregard, never // return, unburden (don't knock it in a mirror, it also cycles), we just have more options with black.
Edit (posted while I posted):
I really can't see Esper standing up to a good aggro list and player without removal in the 2 mana slot. I certainly don't think 3 fatal pushes are enough. Magma spray is also very relevant in the current meta.
3 Impulse
2 Magma Spray
3 Censor's
2 Essence Scatters
3 Harnessed lighting
This was the removal package pre turn 3 we were running ( I was also running 2 BA but that was intended to be used post turn 4 because white is only a splash ). That is hella faster and more consistent than any early removal package Esper can turn out. But can you believe even with this package the Aggro lists were somehow pulling through more often then not. Mind you, it wasn't one sided or anything , the matches were close and weren't easy to play but it was surprising how resilient a good aggro list + skilled player could be against such a aggro hating control deck.
If you're really that worried, black has dead weight, trial of ambition (grasp if you want to rely on BB) and blue has aether meltdown, tightening coils, and even ice over (plus compulsory rest in white, as well as BA like you mentioned). It just think it's super disingenuous to say that esper doesn't have enough answers at CMC 2 or less if it really decides it needs them.