for the fans but also worth trying for newcomers
I've always prefered Metal Slug over Contra. It has always felt more action packed and it has little comedic touches through out the game.

If you're a fan, this is a no brainer. Single Player is awesome, the training challenges are more fun than I expected and the packaging it top notch.

If you're new to side scrolling shooters this may be worth trying. They have included a range of difficulty modes with easy being very easy (in my opinion) and you're not forced to have a steep learning curve like Contra 4.

Best for Fan
If you are a metal slug fan, get it before is tooooo late