I have several phormiums and they're all planted in the ground and i would suggest doing that, they shouldn't need bringing in over winter but you've not filled in the location you live in on your profile, so it would really depend on where you live.
I'm not sure if they do well on clay but south facing they'd like, mine are in thin poor soil and they do really well, some south and some east facing.

If you're going to pot them up it'll need a pot that's about 24" diameter and at least that depth, they've got big roots !

Which Phormium is it? I agree that they're fine outside, mostly, but as Louise says, it rather depends where you live and how freezing it gets in winter. I grow them here in London on clay based soil.