While I haven't faced the new Tau yet, I took a look at the Stormsurge. If you take a Shield Generator, and a couple of cheap systems, The Pulse Drive Cannon [no brainer, to me] and a Burst Cannon / ABFP, you're spending roughly the same points as...
A Knight Paladin with Storm Spear Rockets [Double Tap Battle Cannon, Triple Krak Missile Launcher, and a D-CCW]
or
Wraithknight [Double D-Shot guns]
+ 5x Scatterbikes
So Stormsurge has downright laughable Toughness. And to really crank out the shots, you've got to wait a turn and then NOT MOVE. At which point anything that fears the hundred or so S 5 shots it will put out will get the duck out of Fodge. It can double-tap it's long-range Demolisher Cannon, but again, needs to have an opponent decide to stay in LOS. It has lots of wounds, I'll give it that. But basic marines have a chance to wound with Bolters, so any focused fire is going to start forcing saves. CC potential is Tauish... so nothing to worry about. Depending on whom you talk to, GMC have limited firing capabilities, so the excess of weapons is less amazing than it first seems.
Paladins have a versatile gun that always double-taps. Backed up with the triple krak, has 5 tries with high strength to punch through tough things. Sure, a 2+ save is going to shrug it off, but that's what Swords and Stomps are for. I'd say the Paladin is the better value on the whole. This ignores the benefits of pre-requisite markerlights, but in a vacuum, I think the Pally is the better value. More points is more points, as it goes, so markerlights benefit the Surge, while other points would benefit a Pally. Invisible Knight? Yes, please.
WK is just plain good times, for the price. At max strength, do you really need D-CCW? It's nice, but icing on a cake. It's mobile, ungodly tough, and still lets you take a squad of bikes that can output 20 s 6 shots, that are very difficult to counter. And it's all mobile. WK aren't the scariest thing in the world, but when you compare it to similar-ish models, you find out pretty quickly how cheap it is by comparison, and what the Eldar codex lets you do with 135 points.
No direct experience facing New-Tau, just experience with the last codex. I expect that Riptides are a better value than the Surge.
Yeah, you can buy two Riptides and have 70 points left over for other goodies. I'd take a pair of Riptides over a Surge, at first glance, anyhow.