Dont get any high expectations.
What makes the big difference between Advance Wars and Field Commander is that Field Commander plays extremely slow. Especially the AI isn't half as good as that of Advance Wars, making the game even slower and less challenging.
In Advance wars, the AI always has a nice trick up it's sleeve, making the game as well as challenging as fun. In Field Commander, the opponent doesn't really do that. They choose units rather through the environment of the map then choosing the right units against your units. This makes the game too easy, which is sad, so you're mostly have to gonna choose a map where you are in a significant disadvantage to have a fair fight.
Also, unit classes aren't well balanced either. Not that some are waaay to strong, it's that some are too weak.
Field Commander is full of unnecessary details too, which hardly have any influence on the game itself. For example: You have lots of Co's but they mostly dont differ much from each other.
The campaign is boring, the enemy units are mostly placed randomly. But sometimes the level design is a lot smarter, which are fun. There is a map for example where you are going to lose, so you're gonna let your units retreat. There's one of your units, a truck who has important staff in it, it is vital they survive. Your job is to get them out safely to win. It'll take you more a few times before you know where to put your AA smart, so that their helo's can't reach your truck. On the way, your truck will have all kinds of terrain which may make it slower. Go through the woods so that you can take up more damage? Or take the road which is faster but where you are extremely fragile? Too bad, those kind of maps don't pop up that much.
There are some pluses though. The game itself looks okay and multiplay is fun, and also can be played online. There are also some clever units implanted, for example a soldier who is also equipped with a mortar, so that it can go out on it's own safely, but provides excellent backup.
If you dont own Advance Wars yet should buy Advance Wars. If you dont own a DS either, Field Commander could provide a real TBS fan some enjoyment. Just dont get any high expectations...