Yeah, Stasis has always been the most important Psionic ability in my opinion. A pod with Gatekeeper is much easier when you can first clear the helpers and then focus fire on the big ball on the following turn.
But as I understand in LW2 1.5 every Psi Operative starts with Soulfire, Insanity, and Mind Merge. (And Quick study but that's not a psionic ability.)
Then they can rank up 7 times, at which point they become Master. Assuming
Ufopaedia's info is still correct (the psi unit's page says as of v1.3...), you might want to do some planning, because only 7 new abilities can be learned as opposed to vanilla's everything.
The tiers of abilites are the following. The further down the list the ability is, the longer it gets to train.
Schism | Solace
Soul Steal | Fortress
Stasis | Soul Merge
Fuse | Stasis Shield
Null Lance | Bastion
Void Rift | Domination
If you want to minimize tube time, consider this: If you ever want to learn Domination or Void Rift, you might as well get Solace as soon as you can. For Void rift you need Fortress as well, so you can take that if Solace is not available. The first two rows are pretty quick to learn after the first level-up, but taking Stasis first takes a lot more time (even though you always want Stasis).
My Psi Op perk choices would be the following, assuming they reach max level AND I get these options for training. Training would happen in roughly the listed order to minimize tube time and still have useful abilities (=Stasis) early:
1. Solace, Stasis, Fortress, Fuse, Null Lance, Void Rift, Domination (all good abilities)
2. Schism, Solace, Stasis, Fortress, Null Lance, Void Rift, Domination (replace Fuse with Schism)
3. Solace, Stasis, Soul Merge, Fortress, Null Lance, Void Rift, Domination (replace Fuse with Soul Merge)
I don't care much about Stasis Shield, Soul Steal or Bastion.
If you don't think you want to wait until max level to get all that, you should just select a desirable ability no matter the training time. I don't know how likely it is to get very late skills offered with early ranks, but I don't think it's likely to go Stasis into Null Lance even though that would be possible prerequisite-wise. Null Lance would take a very long time to learn in this case, and it'd probably not be worth it.