Which is better 2550 mAh or 4400 mAh for a mini laptop?

It's not a question about "better", it's about what's most important to you - weight/size or runtime.

The 4400 mAh battery will let you run the laptop on battery longer, but will probably also make it a little bit bigger/heavier. More to carry around.

The 2550 mAh will last a bit shorter, but will keep the laptop smaller and lighter.

You have to decide what's important to you.