OWS gets everything OWE does, minus the ability to choose an XL seat (or any seat designated 'front', iirc). OWS can still select exit row seats for free, and any standard seat. They get the checked bag, fast-track security out of MAD, and priority boarding (in Group 1 with OWEs) too.

I'm not sure if them being on the same booking with an OWE lead booker makes a difference as I've not been in that situation, maybe someone else can weigh in on that specific point.

The only reason I would do this on the IB (or IB Express) websites is it's so much easier to not have to deal with the OTA if anything does occur.

EDIT to note: IB Express website claims free checked bag for OWS only applies to AA/BA Sapphire, but if booked through mainline Iberia this ought to apply to all OWS.