Improved Toughness is good, but like Thanee said, a Psion can't take it until 6th level. Psionic body is available up front.
Psionic Body is a good way to "front load" hp. If a human psion were to take Psionic Body and two other psionic feats (Psicrystal Affinity and Narrow Mind, for example), even with a 10 Con, he starts out with 10 hp.
Plus, while Improved Toughness does provide a hp with each level, for +20 hp at 20th level, it doesn't enhance any other concept of the psion.
If you select ONLY psionic feats (which is possible, see below), at 20th level, a human psion has 13 feats for +26 hp, plus the added psionic benifits of each feat. For non-humans, they still get 12 feats, for a possible +24 hp.
So, Improved Toughness is definately a better feat when looked at one-for-one, but Psionic Body could out-class it in certain situations.
As for what psionic feats to take, make a list of which ones you really want. If none of them require you to expend your psionic focus, then you won't really need Narrow Mind or Psionic Meditation.
You could instead focus on Expanded Knowledge to take some powers not normally available, then add Boost Construct or Metamorphic Transfer (although that would be at a higher level).
If you do plan on using feats that require you to expend your psionic focus (like Power Penetration or Psionic Endowment), then it's even easier to fill out 13 psionic feats.
Here's an example list:
1st level: Psionic Body, Psicrstal Affinity, Psionic Talent (+2 pp)
3rd level: Psionic Talent (+3 pp)
5th level: Power Penetration
6th level: Psionic Meditation
9th level: Expanded Knowledge (bonus 4th level power: Schism)
10th level: Psionic Talent (+4 pp)
12th level: Psionic Endowment
15th level: Greater Psionic Endowment, Expanded Knowledge (bonus 7th level power: Fate of One)
18th level: Psionic Talent (+5 pp)
20th level: Psionic Talent (+6 pp)
Using these (with the notable absence of Combat Manifestation and Narrow Mind), you'd need to make sure you keep your Concentration skills maxed out, and you end up with +26 hp, +20 power points, the ability to manifest two powers a round, and (with Fate of One... you'll be glad you have it if you really need it) a free re-roll here and there.