Zeta Imperial Probe Droid Or Nihilus unique zeta?

They would both be good against an RJT team. The health down means they don't decrease cooldowns as fast, but IPD can completely stop the train in its tracks. If your speed mods are bad I'd say go with Nihilus if your only goal is to help beat RJT teams, since those shouldn't be too bad anyway. But IPD is built to stop RJT. If your mods aren't awful, I would go with that. What do you run? Really jealous that you have IPD.

The problem is that if you are facing a RJT team with zzR2, then the opponents will cleanse the Target Lock every time they crit. Which means that they cleanse it pretty much immediately.
So, in reality, without something like Plague also on the opponents, so that debuffs can't be cleansed, Target Lock is pointless.
(I have Probot zeta'd, so this is based on experience)