OneWorld is not totally seamless. True, you generally get the same credit - one point/mi., or a metric near-equivalent - for flights in any OW program. However, some perks, especially upgrades, are only available (or easier to get) on the airline whose program you belong to. That gives the airline you expect to fly the most an edge.

There are situations where certain categories of flights get different credit in different programs. The extreme case is AA/BA trans-Atlantic, where neither airline gives any credit in the other program. (I'm not blaming them. The reasons have been fully hashed out elsewhere.)

Therefore, it might be a bit short-sighted to base your choice on this one flight. I'd take a look at your future plans too.