Disclaimer
These reviews are going to be as unbias as possible. I rate movies based on how well the plot, characters, emotional pull, scene quality, and creativity work with the movie. I'm doing two reviews in one due to the length of both movies. As said, these movie reviews will be unbias, and also will not count towards the source material in question like the Dragon Ball Z anime or manga. I may reference source materials when need be however.
WARNING possible spoilers WARNING
So the plot to Revenge of Cooler is that Cooler, brother of Freiza, comes to Earth to avenge his brother and his family honor despite hating his brother in general. Cooler invades Earth and faces down with Goku the Saiyen who killed his brother. While the plot is simple, it is done well enough.
The Return of Cooler has a more interesting story too it. Cooler's body didn't burn up completely rather a piece of him survived in space and fused with a powerful computer chip, which Cooler then took control. The computer, now being called a star, has attached itself to new Namek and is trying to eat the planet Galactus style. Goku and friends show up and begin fighting Meta Cooler and his robot minions. Then Vegeta shows up to kind of help. The plot here is really good honestly and works well with both the movie and DBZ lore in general.
Really, the only character we see building well in either of these movies is oddly Cooler. Cooler actually has a great deal of character development in both movies, that being said, the other characters even Goku and Vegeta don't really build too much as characters from either movie. Cooler is a good character honestly, but we don't see much of the other characters, especially in the second movie, however we do see some okay character building by Goku in the first of the movies but not much.
Thankfully both movies have pretty decent action and Cooler made both movies somewhat perilous. There were some awesome moments where Goku and Gohan had to hide from Cooler's men in the first movie, as well as someone nearly killing all the magic beans. The second movie does this well too by having an entire planet being nearly eaten and having characters captured and jailed, even if the characters never developed or had a large standing with the plot. Great movies for action junkies and suspense lovers.
Revenge of Cooler was great, while older its animation is excellent and the scenes flowed really well together. Nothing seemed out of place and the fights weren't overly confusing. That being said, Return of Cooler obviously had some budget problems. The animation doesn't hold up as well, and some of the scenes look like the concept art was taped into the scene for some reason. Also there are several scenes that just seem cheap and not as well done. It wasn't bad and flowed nicely, and the sound was okay, but the animation seemed clunky in some areas.
I like Freiza's design, but Cooler's design I feel is far better. His newer transformation is also really well designed and awesome. I don't mind his henchmen but they are kind of less unique in the movie. For the second movie, Meta Cooler's new powers and the reasoning for them are pretty cool, and Meta Cooler's design as well as Cooler's final wired form are well designed, and the creativity of the star is amazing as well. Both movies have a great amount of creativity, and flaws with the lackluster minions. Does Cooler make up for it? Yes he does.
If you like plot more then anything, Return of Cooler is your movie, however if you like animation better then anything in anime movies, Revenge of Cooler is better for you. If you are an action junkie, both movies will feed your gullet for anime action. If you are a fan of Dragon Ball Z and know of Frieza, this is a great movie for you.
Honestly, both movies have strengths and weaknesses. Outside of Cooler, neither movie does great with the other characters, the plot in Revenge is simple and okay, the animation quality is not as good in Return, and everything else is great in the movies, though it doesn't introduce the characters all that well, so I only recommend watching if you know some of the characters of Dragon Ball Z, particularly Frieza.