I'm at base lvl 10, Raider class, 220,000 troops. I'm given the opportunity to change class before..

This will depend on whether you are enjoying playing as a Raider. The two classes are not hugely different. But as a farmer you will get better rss production and some protection from loosing resources if you are attacked. As a Raider you will be stronger when you attack and can take more from other.

If you are happy playing to be a strong fighter and like to spend time attacking and raiding other players then stay as you are. If you have less time for that and would prefer a more passive game then switch to farmer.

You will get opportunities to change class every 3 levels, so you will be asked again at level 13, 16 etc.