Like many manga published in Shonen Ace, the manga of Brigadoon: Marin to Melan was created to promote the anime and was based on an early script of it, and because of that it's hard to judge it on its own merit. If you do read it standalone, it's a sweet yet forgettable mecha story about a girl and a robot. In some ways, it answers the biggest issues with the anime: the fanservice is more toned down, and the ending is less convenient and convoluted. A common yet objective criticism of the anime is that it's too tonally dissonant, and if you found this anissue, you might prefer the manga- it's both less comedic and less dramatic and violent.
It has good illustrations for the most part and the manga's alternative backstory for Marine is interesting, too.

But perhaps because it tries to compress 26 episodes into 2 short volumes, almost every other element of the series is done better in the anime: in the manga, the jokes don't hit as well, the fights aren't as well choreographed, the themes are more heavy-handed and less poignant and the story beats are more predictable.

The most egregious changes regard the characters themselves: it's common for them to act in mean-spirited ways, where in the anime even when people do terrible things, it's always shown that they're motivated by misconceptions and pain, rather than innate evil. Marin and Melan's character development is rushed, and so is their relationship to each other: them being in love is told to us by other characters rather than the reader seeing it from their interactions.

If you were a big fan of the anime and want to see an alternative version of the story, I can recommend this manga. But for anyone else, it's something you can pass on.