OK, here's the comparison table between a Black Ethergaunt and a level 20 Wizard. Let's assume there's a 32-point buy, the Black Ethergaunt and the Wizard both buy Str 8/Dex 14/Con 14/Int 18/Wis 12/Cha 8. At levels 4, 8, 12, 16 and 20 the Wizard bumps Int. On the other hand, the Black Ethergaunt does not get any ability score increase from its 16 RHD, as these are already included in its ability scores. Let's also assume they both have bought a Tome of Clear Thought +5 and a Headband of Intellect +6.
black ethergaunt level 20 wizard Ability scores Str 10 Dex 22 Con 18 Int 49 Wis 16 Cha 12 Str 8 Dex 14 Con 14 Int 34 Wis 12 Cha 8 average HP 168+(16*Con bonus from items) 90+(20*Con bonus from items) max spell level 9th 9th spellslots (0th/1st/2nd/3rd/4th/5th/6th/7th/8th/9th) 4/9/9/9/8/8/8/7/5/4 4/7/7/7/7/6/6/6/6/5 spell save DC 29+spell level 22+spell level feats/bonus feats from wizard level 6 7/5 Base saves +5/+5/+10 +6/+6/+12
Alright, here's my mindset: a player willing to use a Black Ethergaunt as a PC is at least a moderate optimizer. Hence, the would-be Ethergaunt PC has to be compared to an at least moderately optimized Wizard 20. For this reason, I think putting Shapechange on the table is very relevant, and not at all theoretical play. Given that the Wizard 20 has one extra 9th level spell per day compared to the Black Ethergaunt, let's assume he uses this spell to cast Shapechange. Then, we have two possibilities.
1. He finds some form to change into that's even better than a Black Ethergaunt (say, a Chronotyryn). In which case, the logical thing to do for the Black Ethergaunt PC is to also use Shapechange to turn into said Chronotyryn. The Wizard is thus still one 9th-level spell ahead for the day. (Except if said form has too much HD for the caster level of the Ethergaunt PC, but not too much for the Wizard PC, but given enough CL tricks and spent WBL the Ethergaunt PC can quite likely overcome the difference)
2. He turns into a Black Ethergaunt. The Black Ethergaunt PC does not, and the two PCs now have an equal number of 9th level spells for the day.
Outcome 1. is quite more likely, given that better forms than the Ethergaunt exist. Let's suppose that's what happens.
The two PCs will end up with the exact same array of physical ability scores, movement modes, perception modes, bonus to AC, (Ex) and (Su) abilities. For this reason, their AC will be the same, and their save modifiers will be exactly the same (with the exception of Will save being a tad higher for the Ethergaunt) and applied on almost identical base saving throws (the Wizard's being a little better) will yield almost the same final saving modifiers. Skill points are a nonargument when discussing the power level of two high-level wizards. We're done with these.
The Wizard has more CL, the Ethergaunt more HPs. In order to reach CL 20, let's say the Ethergaunt takes Iron Will and Reserves of Strength, and routinely pumps its CL by 3 each time it's needed. The 5d6 damage it takes will be offset by the extra 78 HPs it has over the Wizard. So that's settled too.
As far as spells per day go, the Wizard has one more 9th-level spell and one more 8th-level spell; the Ethergaunt boasts a massive 2 extra spells per day of each level from levels 1 to 3, 5 to 6, and one extra at levels 4 and 7. Let's convert that into Pearls of Power. The Black Ethergaunt needs to spend 64+81=145k gp. The Wizard needs to spend 2*(1+4+9+25+36)+(16+49)=215k gp. Meaning the Ethergaunt comes out on top with an extra 70k gp to boot.
But then, the Ethergaunt only has 6 feats, 2 of which are already taken by Iron Will and Reserves of Strength (let's say it gets Iron Will from a magical location, so that's only 1 down; on the other hand, the Wizard gets to DCFS away his bonus Scribe Scroll to something more useful it it wishes so). So that's 5 open feats for the Ethergaunt vs 12 open feats for the Wizard. The Ethergaunt blows one feat on Obtain Familiar, to match the Wizard's. It's now down to 4 open feats. The Wizard is in excess of 8 feats.
However, the Ethergaunt's spells' save DCs are a whopping 7 points above the Wizard's. How can the Wizard overcome that? Well, there's the Irresistible Spell metamagic feat (which needs two other feats as prerequisites), from Kingdoms of Kalamar, that adds an extra +10 to save DCs, at the cost of a +4 level adjustement. The Wizard can then take Southern Magician to be allowed to cast once per day per two caster levels any spell as a divine spell; he takes Planar Touchstone (Catalogues of Enlightenment) [Sun domain from Dark Sun Campaign Setting]: that grants him Turn Undead. He finally takes Divine Metamagic (Irresistible Spell), and buys a bunch of Nightsticks. There we go. The Wizard has used up 6 feat slots, and a bunch of money into Nightsticks. Let's say he spends half of all the money normally used for consumables on them (which is, as a rule of thumb, 25% of one's WBL). Meaning he uses up, at level 20, 95k gp on them (that's about 13 Nightsticks). Sure, the Wizard can only do this a couple times per day, whereas the Ethergaunt can always do this and for free; but when the Wizard does pull off the trick, the save DC of his spells ends up being 3 higher than the Ethergaunt's, so I'd say this roughly balances out.
When all is said and done, on the one hand the Ethergaunt gets an extra 70+95=165kp gp (and more than 100 extra skill points, for that matter); on the other hand the Wizard still has 2 extra feats. The Wizard has pulled off quite a complicated combination of feats from three different campaign settings, two of which are quite obscure, has made a questionable use of Nightsticks, has dug off a Shapechange form better than the Ethergaunt's...and is 2 feats ahead and 165k gp down.
On the other hand, the Wizard could choose not to try and mimick the Ethergaunt and do completely different things with all his feats, take powerful PrCs instead of a bland Wizard 20, etc. If he tries to not be a Black Ethergaunt, he can - whereas the Black Ethergaunt cannot untake his RHD to take levels in, say, Mindbender, or Dread Witch, or Fatespinner, or Cerebremancer, or...so that, more or less, balances things out in my mind.
My verdict: on the whole, the Black Ethergaunt is far from being put to shame by the comparison. But it's not a landslide victory by any means, either. In regard to this, I vote LA +4 for the Black Ethergaunt. Incidentally, I've also voted, for the Red Ethergaunt, for an LA that would put it 3 caster levels behind a Wizard of equal ECL.
It has taken me ages to do all of that, so I can't be bothered to do the same for the White Ethergaunt. I'll just suppose the "3 CLs above equal ECL Wizard" holds true there too, and as such vote for LA +5 for the White Ethergaunt.