for a player on a bosman? that's ~19m gross (without the new tax rule, which is about to be revoked), more or less the same amount as arthur cost us per season. for the same money, i'd certainly pick alaba out of these two.
we only have two players who are clearly better on their respective positions than alaba: cr and de ligt. both cost much more than 19m. ffs, douglas costa did cost ~22m last season. i wouldn't call alaba a bargain by any means, but if those rumored numbers are accurate, he's not excessively expensive either.