just wondering bc i always thought they were the same person

Imperiex Prime is a title used to differentiate between the real Imperiex, and numerous Imperiex probes, which are just like him, only much smaller and much less powerful.

the probes are slightly larger than a normal human, while Imperiex Prime is roughly the size of Galactus.

o thats what i thought but i couldnt verify that from their descriptions

Also their goals differ, the Anti Monitor is out to convert everything into the Anti-matter universe while Imprex is out to wipe out anything it deems 'unpure'. Imprex detected something about the DCU milkeyway that warrented its attention and came running, finding out in the end before it was killed that it was actually itself and the Brainiac-Warworld it had detected.

It's definitely debatable about which one is more powerful (Mandrakk or the Anti-Monitor). I would say that Mandrakks opponents was unquestionably more powerful combined but the Anti-monitor hasnt had it easy either and keeps coming back
 
 
 
Mandrakk was stopped by the combined power of Superman in the Thought robot (made of pure thought and capable of adapting instantly to counter any future threat. A sentinel suit designed for the single Purpose of Protecting All Existence Against The Ultimate Enemy), the combined power of the Supermen of the Multiverse, Nix Uotan, angels of Pax Dei, the Forever People, and several Green Lanterns. 
  
 
 
 
 

Anti-Monitor    
 
1st Time - The Monitor with aid of the Harbinger gathered these heroes from different times and different worlds to bring down the Anti-Monitor. Psycho-Pirate, Supergirl, Flash: Barry Allen, Superman of Earth-Two, Superboy-Prime and Alexander Luthor of Earth-Three helped in his defeat the first time.
     
2nd Time -  Sodam Yat, the Guardians of the Universe, Green Lanterns Guy Gardner and John Stewart, and Superboy-Prime all had a hand in defeating him the second time. 
  
3rd Time - formed a bullet around Dawn Granger and place her into a giant sniper rifle construct.  When firing the bullet, all of the Lanterns present (of every corps color) would douse the bullet with their light and by doing so would effectively take out The Anti-Monitor for good.  For the first time since the original Crisis, The Anti-Monitor had not only been defeated but killed in the process as well.     
 
 
 
 
 
Imperiex fought the hero's and villain's of the world as they united as one to fight him off.  Kismet and Superman managed to crack Imperiex Prime's armor, but Kismet was killed during tthe attack. Superman, with help from many other heroes, manages to send Imperiex Prime's consciousness back 14 billion years.