Which Palkia ?

If you have a lot of stardust, power up the 98.
If not, power up the 89.
Though if you have to ask this, probably the second option.
BTW the 98 is always the better option if you can power it up.

What ^he said. If you don't have the stardust to make asking this a non-issue then always go with the lucky.

Neither. After 4:00 PST, there's no strong use for a Palkia. Wait until you have a use for it. Of course, if you want it to head up your Dragonite raid team, well, that's a use. In that case, what Gregory said.