Save your certs for C4; no contest.
Partly because C4 is great for blowing stuff up; I especially like LA + C4 for difficult to shoot Sunderers that are hidden away behind a building - off you go over the rooftops to drop 2x C4 on it. Even if I don't get the kill, it will be criticaly damaged; easy for a team mate to finish off. Can really rip the heart out of an attack. And as Neutral said, good for MAXes too (c4 my commonest cause of death when I'm running as a MAX
).
And partly because in PS2 dying rarely matters, as you can usually respawn nearby. I never bother with a medikit.
Tip - if you are badly wounded and an equipment terminal is nearby, go to the terminal and switch to Medic. Heal yourself and then switch back to your original class.