Manga

Outside some minor changes that come from this being an adaptaion (player commander being replaced with an actual character, expansion on what happens between and during missions etc) it's pretty faithful. Definitely way closer to the game than the anime was... (￢_￢)

There's also additional characterization of all of the units which I appreciate - game was a lot less expansive on that front in the early days.

They don't seem to be doing any actual filler if that's what you're worried about but don't expect an ending anytime soon. While we're entering the final arc in the game, we don't know how many events it'll take to actually finish the story. Manga is currently at event 3 out of 15 (?) major ones and doesn't update that often so it'll be a while before it reaches game parity - especially since later events got quite a bit longer.

I think I read that release schedule might improve but don't quote me on it as I need to confirm whether that comment was about this series or one of the spin-off ones.

All in all, I'd say it's a pretty good alternative if you don't want to play the game. There's plenty of good story waiting to be adapted, you just have to hope we won't die of old age before it happens.

Edit: I'd also like to add that translator of this series tends to add joke'y comments on some of the panels. They are easy to recognize and kind of hidden but don't be surprised if you see something that doesn't necessarily fit the mood. Unfortunately, there's no alternative in this case.