Port forwarding does matter to increase ypur availability. It is not a requirement to be able to seed and you can also seed at pretty decent rates without port forwarding, though. I am doint it with mullvad without PF and it's fine.
In my opinion, there are no other VPNs with a similar quality of mullvad that support PF at the price of mullvad.
At the time I tried (2009), I was too young and afraid of messing up my Windows (I didn't even know if I was able to remove Windows and not loose the warranty), so I was finally did thanks to the marvelous Wubi installer. It let you try Ubuntu without messing around with partitions. Thanks to that, I was able to start learning Linux until I gained confidence and did a proper dual boot.
Now I only use Arch, even for work.