I have used both Mullvad and AirVPN for extended periods of time.
Mullvad is the gold standard. If you don't need port forwarding and privacy is your biggest concern there is no better choice. Check out their server infrastructure, and tools such as DAITA / Leta / Mullvad browser to see how much they contribute to the privacy community.
Mullvad is also a much better UX. AirVPN Eddie client feels like its stuck in 1998.
Both are good VPNs - Mullvad just happens to be the best. The only real issue, from a privacy perspective, with AirVPN is they refuse to be audited so you are stuck just trusting they do what they say but OTOH they have been around for a long time and never given a reason not to be trusted.
If you do need port forwarding, AirVPN probably has the best implementation of it. They also have customizable DNS on a per device level which is also pretty cool.