Nick,
Most Dive Centres / Filling Stations charge the same to fill any size Bottle.
300 Bar , Gives more fills than 207 / 232.
Even if local centre can not fill to 300 Bar , the few £'s @ time of purchase will be worth it in the long run.
Especially if you get a MDE Air Gun Valve fitted to the Bottle to Start with, as they only need testing every Five years.
As can not be used in Water, so should not rust like water used bottles.
There has been posts , which explain the fills per bottle.
But talking about many times more with larger Bottle.
I have 12 Litre 300 Bar , as main bottle and 3 Litre for running my Ram Charger ( Brocock Tacs ) , so that I can control the preasure in my Ram Charger.
But my 12 Litre , used to give me at least 20 odd Rifle Fills , for my assorted Rifles.
Not only Fill cost around £4.00 per fill , but also the time taken in getting Bottle filled.
BOB/R