Fermenting under pressure/head space

[MFigz]

I'm planning on stepping up from buckets to a conical, and I've read about the fermentasaraus Gen 2 which sounds amazing, especially for the price! I typically brew 5 gallon batches but occasionally will brew a 10 gallon batch. I was originally looking into 14 gallon conicals so I could do either batch in them, but then I read that too much head space was not good for the fermentation. Then I also read that when fermenting under pressure (which sounds like a great idea with the fermentasauras) the head space doesn't make as much of a difference. One other consideration I have is that I can fit the 7 gal fermentasauras into my fermenting chamber, but not the 14 gal without some major modifications (but still doable).

I'm having a little buyers paralasis right now, any opinions on whether I should go with the 7 or 14 gal(ish) fermentasauraus?

Thanks!
Mike

 

[Sadu]

Brewing 5-6 gallons in the big fermentasaurus will be fine. The headspace won't cause any problems in the pressure environment so long as you don't do anything silly like opening the lid.
So really it comes down to what size batches you are brewing and whether you can be bothered modding that fridge.