In parallel runway ops, is it better to dedicate a single runway to landings, or is it best to have a mix?
That would no doubt depend on a lot of factors. Geometry for example. If the parallels are far enough apart there's no chance of a missed approach on one interfering with traffic on the other, you're a lot more flexible than when they're close together.
Maybe you have noise abatement procedures that make the use of one of the runways for either landing or departures illegal at specific times (e.g. Schiphol can't use certain approaches during the morning rush because overflight of parts of the city are not allowed at those times).
Type of traffic is another. If you're getting a lot of inbound heavies at an hour where you have a lot of outgoing smaller aircraft, it soon becomes a good idea to have one runway set aside during that period for landings, one for arrivals (the wake turbulence of the heavies would mean longer waiting times on the runway for the lighter aircraft).
So overall, there's no definite answer. It all depends on traffic, local conditions, etc. etc.