My personal preference would be Kratom. Depends on the circumstances because Kratom is something I can take all day long and have excellent results, Phenibut is something I only take in the evening when I'm not doing anything. They are relatively inexpensive so I say try both and see what works for you. If you have questions on Kratom specifically there is a Facebook group called "Kratom Motivation" you can check in with
Phenibut I take for social situations.. Like before meetings. It helps make me more social/be able to articulate better. Kratom I take a bit after lunch to fight off the drowsiness that occurs in the afternoon, although I meditate a bit to help with that as well, and then also before working out. Kratom is something you don't want to take "too much" because you grow a tolerance to it relatively easily. Mind you, you want some tolerance if you're taking it at work.
In the Kratom community, Phenibut is taboo. Most users advise against if strongly, because the benefit isn't equal to the risk. It is supposedly very addictive, and because the effects are subtle, one can take too large of a dose. The withdrawal can be murderous. This could be hearsay, but I've heard enough horror stories from people I trust to abstain for now. The risk isn't worth it, IMO. Kratom, on the other hand, has been a wonderful remedy for me. Pain relief, focus, well being, clean energy, and no side effects if taken properly. Withdrawal is mild, if any
Phenibut is a once or twice a week thing or tolerance and withdrawals are common. It's GABAergic and acts primarily on GABAb
Kratom is very different and works primarily on opioid receptors for analgesic effects and indirectly can increase dopamine for an energizing effect. Done find a mild anxiolytic effect as well.