Both would be suitable. However, I'd go for Berkshires. Easy to handle, reach pork weight by five months old, and very economical to raise. GOS are extremely popular, but friends who have them find them hugely stubborn when it comes to moving them from a to b - e.g. loading them onto the trailer for the abattoir. Also, Berkies won't suffer from sunburn like GOS over the summer months (should we have a summer this year!).
Don't fret over it - choose one breed (flip a coin if you have to), take them to pork weight. Next time, try another. That's the only way to find out what pigs/pork you prefer. Have fun and enjoy your pigs x