They are too different players to say who was superior.
They played too different roles for their teams.
As pure talent it's Ruzicka, I guess. He is also significantly ahead of Zhluktov offensively.
To compare them is like compare... I don't know... Toews and Tavares, probably... Zhluktov never had Toews' leadership qualities though... But, probably you got what I mean...
Zhluktov was always a glue guy in his lines. He was great defensively and absolutely one of the top PKers I've ever seen. He was good in forecheck and in front of the net. He was decent scorer and play-maker, but that was not his stronger sides.
It's pretty telling, that he was tried as Soviet #1 C in late 70th, beginning of 80th then it became obvious, that KPM' days were coming to their end. The new #1 Soviet line, that should've change KPM line, should've been Kapustin - Zhluktov - Balderis line.
It's pretty telling that they failed in this role.
So, you can't have Zhluktov as a player to built you line around in term of offense. But if you have good scoring wingers he will be perfect fit.
On the contrary, Ruzicka doesn't do anything else but offence, but it's hi-end offence, no doubt. And he can provide it by himself. So, you CAN have Ruzichka as your line main offensive threat, but you'll need somebody to play defense and somebody to do some dirty work in corners/along borders to have your line successful.