Repotting Phals - pot size

[Lauchi]

Hi!
I have two young phals that have quite healthy root balls, each in a 12cm (4,7 inches) pot.
I will be repotting them soon, and have been wondering whether to repot into a 15cm (5,9) or a 17cm (6,7 inches) pot.
The 15cm seems more reasonable at first, specially since the plastic nursery pots are quite cheap, but the "outside pot" is a bit more expensive, so I would want to buy a size that is going to last more than a few months.

I live in a dry climate right now (Madrid), so root rot is less common than dehydration for my plants in general.

In your experience, do average phals do well in 15 cm pots for a long time? Are larger pots (17cm +) usually needed or is it more an exception than a normal thing? What pot size do you have most of your phals in?