I'll try not to bring back the whole "TTGL vs. Getter Emperor debate" into question, however, I dare to say IMO Emperor is more powerful. Reason?
-TTGL, despite the claims, wasn't really "big enough to walk on galaxy". It was huge, yes. But the universe in which it fought against Grand Zamboa was artificially created pocket space where thought was given form-nothing was real per se, it was also smaller, and destroying Anti-Spiral Homeworld would bring down the whole illusion. Also, Anti-Spiral Homeworld was a planet roughly Earth's size, and it could be clearly seen on Grand Zamboa's forehead. On the other hand, Emperor starts off as three planet-sized ships that can combine into a gigantic mecha. And said mecha can keep on growing larger and larger, as it absorbs Getter Rays. In the end it's big enough to have entire galaxy twirling around it's waist.
-people claim TTGL was so strong it could attack anything at any point in time and space. Kinda wrong, as only Super Galaxy Gurren Lagann was shown to be capable of doing this. Only thing we saw TTGL be capable of doing is creating weapons out of nothing. Getter Emperor on the other hand is capable of terraforming planets (or destroying them with ease), destroying time-and-space distortions, and even create similar things in order to summon other Getter Warriors from other multiverses (as every Getter Team's fate is to in the end join the Emperor in the eternal battle against the enemy). Also, Emperor technically becomes the physical embodiement of evolution itself-whichever it deems worthy will evolve to high levels in a small ammount of time.
-TTGL had great difficulty of defeating the embodiement of a collective mind of Anti-Spirals, until Lordgenome sacrificed himself to give them a power-up. Even then, all of the mechs were wrecked, only Lagann finally managed to kill AS by ramming through him, not before being severely damaged itself. Getter Emperor one-hit killed a god-like being by smashing her head with his fist, turning her head into a galaxy as a result, there's no telling what ammount of rape would he deliver to some collective-minded drone.

But anyways....why the whole goddamn "this mecha would beat the other one" debate? The two would just join forces and ruin some villain's life big time. And be an amazing team-up while doing so, period. :P (Lick)

Getter Emperor belongs to Ken Ishikawa and Dynamic Pro.
Art by me.