What to buy 600 mm or 700 mm

PRO - I can start with the main one, the longer the barrel the more power.....
CON - Longer barrel on some guns makes it longer when carrying
PRO - More power, can shoot heavy and lighter things faster
CON - With a mod on the end its longer than 700mmm

What it says above.
For me 34" is good 38" not so good, but that is because for me it would mainly be a field gun, so "for me" it is worth the trade off on power/shot count. your use/needs may differ.
good luck

I agree with above as well.
The nice thing about Airgun's as well as PB manufacturing today is the interchangeability of so many components and upgrades that are available from the factory's and 3rd party vendors. In many cases ( as we have seen in the AR market) you add a few parts for a few hundred bucks and you have a gun with a new purpose. It's doesn't have to be this one or a that one, you could have both. How cool is that!

Well I’ll be going for the 700mm on the Crown Continuum with a donnyfl making it 800mm. Reasoning is as Giles says more power and with a slug liner I’ll be happy enough. Only thing that sucks is here in Northern Ireland you can only have One barrel not both the 380mm and 700mm that ships with the Continuum. So I’ll go with the best option the 700mm.