Okay, so you might notice that all of the screenshots for this reciew are of the game's first stage, and usually, I wouldn't review a game if that's all I could see of it, but this is a special case. Most of my time playing this game was the DSiWare version which I have on my n3DS. I don't have any way of taking screenshots of DSiWare games, though, but I knew the game was released as a regular DS cartridge, so I played that too, for screenshotting purposes. It turns out, the two versions aren't the same!
The basic core of the game is the same, of course: it's a pretty decent Metal Slug clone. You go through stages, shooting lots of guys, and occasionally robots and tanks and helicopters. There's a whole bunch of temporary power-up weapons, destructible parts of the stages with hidden items, and most other Metal Slug things. Except that vehicles only appear in special seperate substages, not as thing you can just climb aboard in the regular stages. It's pretty good. Obviously nowhere near as good as the first few Metal Slugs, but maybe in the same league as the later entries.
The differences between the two versions are all related to difficulty. The stages in the cartridge version are a lot longer, to the extent that entire sections are removed from the digital version. Of course, I've only seen the first stage of the cartridge version, but the first stage is made up of three sections: one on a jetbike in a forest, one outside in the snow, and another inside an underground military base. The digital version of the first stage, meanwhile, and the indoors section is slightly truncated, as well as the larger gatekeeper enemies having significantly less HP. The enemies in the digital version generally have less HP, and they inflict less damage on the player, too.
Which one is better? To be honest, there's no perfect version. The cartridge version is a bit of a chore: I could definitely overlook the difficulty if the stages were shorter, and the bigger enemies didn't feel like such bullet sponges, and while I definitely prefer the digital version's faster pace, it also feels like there's no challenge at all, and most of the times I've played it, I've actually gotten bored and stopped playing the game long before I came close to dying.
So I guess the only conclusion is that Commando: Steel Disaster is a game with two different versions on the same platform, both of which are terribly flawed, in almost completely opposite ways. Just play Metal Slug X instead, is the best advice I can give here.