hi,

iam wondering what to invest in.

If i only invest in training facility, and dont invest in regeneration, what is the downside of that?

And are training and regen level suppose to follow eachother, or does that not matter?

You should keep those both at the same level. Traibing facilities increase training, but if you don't have appropriate regen the players will lose energy.

You can have regeneration area little lower as training area (max 19levels) otherwise your players do not regenrate fully and lose energy.