As far as Nix vrs REB
I think the biggest card is Drain. And if your in a drain meta then the arguement is clear.
Brainstorm could be good ... but how often do you really Reb brainstorm
Gifts and FoF - generally are not going to be cast without back up. Also they are expensive, so your most likely looking at FoW or MisD as back up -or- REB or even Nix as back up. So in order to stop Gifts or FoF you probably will need double counterspell. Double REB will work, where double Nix will not. Outside of that your looking at needing a drain or force or your own to stop either of these spells, and your using Nix/REB to back-up.
Interestingly enough, you can REB a Nix, but you can't Nix a REB (that sounds like the start of an IQ test question), so in a Nix heavy meta, REB would be better.
I think I like Nix because Nix hits at least a few cards in every deck. Even Mono Black, Shops, TMWA, blue-less Belcher, ... every deck has at least lotus and some on color mox. So as far as a Maindeck slot, it's better than REB. If were talking board slot then REB is better.
If you're not considering REB'ing Brainstorm early game against X-Long variety or GAT, then your thinking is vastly different than mine (as in I think you stop the engine before it gets started).
Mono-black, TMWA?? I guess if that's what your meta is like, REB won't be too good, but against a field where the popular and top table decks are X-Long, Flash, GAT, Fish, along with Shops and then randoms, REB is CLEARLY more IMPACTFUL against more top-tier decks and more cards.
Let's just go through 6of the top 7 topics for top8s and tournament reports here at TMD:
Moxpearls report from COD - he played Flash Slivers, Bomberman, Long, Flash, UW, Slaver, Long, ICBM Oath. REB would be better in EVERY single one of those matchups.
Let's look at the untouchables Top 8 - UR Landstill x2, GAT, UW Fish, GAT, Infernal contract Long (lots of Blue), Stax, BW Fish. REB is a MUCH BIGGER beating vs 6 of the 8 decks and misses two. The only advantage NIX has is countering some Mox or Chalice for 0. That's losing out against EVERY SINGLE CARD except crucible in the Landstill, most of the GAT decks, and all the non-Fow spells except STP but including meddling mage vs UW Fish.
moxpearls report from Monster den - staxless stax, 5c stax, ub fish, bomberman, gat, uwb fish, - again REB is MUCH better in 4/6 matchups and in the other 2, you get to counter a mox or chalice 0.
some other guy's report from RIW - Ichorid, Oath, belcher, oath, mostly blue deck, oath, combo, ubw fish. Again, REB is better in 6/8 matchups with the only SIGNIFICANT advantage for Nix being Ichorid (Therapy).
God Campbell's report form Untouchables - stax, metalworker/shop, urphid, bomberman, GAT, UR landstill, ur landstill. Again, REB is MUCH better in 5/7 matchups and all nix does vs stax and the other shop decks is counter a few mox, maybe chalice 0.
Tin Mox report from the meandeck open - GAT, the meandeck, GAT, slaver. REB is much better against all 4 decks than NIX.
That's just 6 of the top 7 topics in the Reports forum at this given moment (removing the tokyo metagame aside because they are small and not that mature of a metagame). So basically for all the top 8s and/or reports, against those matchups/decks, REB is considerably better than NIX an OVERWHELMING majority of the time.
Maybe it's just me, but the evidence seems overwhelmingly in REBs favor at this time, assuming your deck can support red.