We're Europeans making a movie with the HV20 and I'm wondering if we should just slow down all our footage from 25P to 24P before editing to make it as filmic as possible. It fits in our workflow perfectly because HDLink can do this upon capturing/converting.

It seems to me that 24P is a slightly bigger standard than 25P and also makes it easier to convert to film etc. However, I read somewhere that there may be problems converting it back to 25P for European tv's in the end, is this true?

I would stay with 25p.
The difference between 24p and 25p is almost impossible to see, and many theatres in Europe project in 25p.

IF you would need 24p, you can always do that conversion later. But for convience, stick to 25p.

If you do decide to go from 25p to 24P, make sure you increase the pitch of your audio by 72 cents.

Stick with 25P acquisition if you also plan on showing it in Europe and other PAL territories.