Frank can ward off Plasma Weapons easily, and punched a Deathclaw in half apparently, but Liberty Prime was a walking juggernaut, taking down Vertibirds in single shots and mowing down the Enclave with ease.
Also, Liberty Prime was much larger, Frank is about 12 feet tall, Liberty Prime is forty.
Hell, Liberty Prime even has a personal catch of Mini Nukes it can throw like grenades, although his mini nukes are larger.
And then there are LP's awesome(and hilarious) quotes.
"COMMUNISM... IS A LIE."
"DEATH... IS A PREFERABLE ALTERNATIVE... TO COMMUNISM."
"DEMOCRACY... IS NON-NEGOTIABLE."
"COMMUNISM... IS THE VERY DEFINITION OF FAILURE."
"CHAIRMAN CHENG WILL FAIL...CHINA WILL FALL!"
"VOICE MODULE ONLINE... AUDIO FUNCTIONALITY TEST... INITIALIZED. DESIGNATION: LIBERTY PRIME. MISSION: THE LIBERATION OF ANCHORAGE, ALASKA. PRIMARY TARGETS: ANY AND ALL RED CHINESE INVADERS. EMERGENCY COMMUNIST ACQUISITION DIRECTIVE: IMMEDIATE SELF-DESTRUCT. BETTER DEAD... THAN RED!!"
"FREEDOM... IS THE SOVEREIGN RIGHT OF EVERY AMERICAN."
"EMBRACE DEMOCRACY, OR YOU WILL BE ERADICATED."
"ENGAGING RED CHINESE AGGRESSORS."
"ANCHORAGE WILL BE LIBERATED."
"COMMENCING TACTICAL ASSESSMENT: RED CHINESE THREAT DETECTED."
"COMMUNISTS DETECTED ON AMERICAN SOIL. LETHAL FORCE ENGAGED."
"COMMUNISM IS A SMALL SETBACK... ON THE ROAD TO FREEDOM."
"OBSTRUCTION DETECTED. COMPOSITION: TITANIUM ALLOY SUPPLEMENTED BY PHOTONIC RESONANCE BARRIER. PROBABILITY OF MISSION HINDRANCE... ZERO PERCENT!!"
"INITIATING DIRECTIVE #7395: DESTROY ALL COMMUNISTS!"
"THE LAST DOMINO FALLS HERE. COMMUNISM... IS A LIE"
"A MINOR SETBACK ON THE ROAD TO FREEDOM."
"COMMUNISM... IS DEATH."
That said, all my info of Frank Horrigan is from wiki, so this post will most likely be ravage to death by someone who has knowledge on both characters.