We can use 24L/R simultaneously if one (the inner, so 24R in this case) is used exclusively for takeoffs and the outer is used exclusively for arrivals. (We can't run simultaneous arrivals on 24L and R, as the runways are too close for that.) That would actually be the preferred configuration: 24R departures only, 24L arrivals only, 23 mixed-mode (both). Same thing in east operations, use 5 in mixed mode, 6L departures, 6R arrivals. Someday, years from now, we'll add a fourth parallel south of the current 05/23 and have two sets of parallel runways to use, similar to LAX, DFW etc..
For now, in order to "triple" (use all three runways as I described), we need extra staffing in both tower and TCU, so the reality is that this is usually only doable during certain hours during the afternoon and evening, and sometimes not even then depending on how many bodies are available.
More often, we prefer to use the parallels in a "dualling" configuration, in which both 05 and 06L (or 24R and 23) are employed in mixed-mode. This is more efficient than having one runway be a dedicated departure runway and another be dedicated for arrivals, since it's rare that the number of arrivals and departures in a given period of time balance out exactly. Also, it's a long taxi from one end of the field to the other, so having both runways available for departures and arrivals helps to minimize aircraft having to do that.
As to which of the south parallels gets used in that type of operation, it will generally be the inner (6L/24R). It's longer, as well as being a shorter taxi from the terminals. However, it depends on the runway surface condition, whether all approach aids and lights etc. are serviceable, whether maintenance is being done, and so on. As far as I know, there's no operational advantage to using the outer in this type of operation if everything else is equal, but I don't work terminal/tower so it's possible that there's another factor that I'm not aware of. I'm honestly not all that up on that particular end of things, literally, as the arrivals I work almost always go to 05/23 if it is open.
When we go onto 15L/R or 33L/R, that's when the operation slows down since we can't run simultaneous approaches to both runways (again due to proximity). This means, again, that one runway is departure-only and one is arrival-only, which is less efficient. If this is happening during peak hours there will almost for sure be holding involved.
Then you have a fourth, non-parallel type of configuration (eg. depart 33L, land 6L) which is sometimes used late at night, for noise abatement, or due to maintenance or snow clearing operations. This is another situation we try to avoid during peak hours.
Hope this helps, if not I think there's somebody from the tower on the forums as well and he would be the expert on runway selection. The enroute guys, and even the TCU, don't select the runway configuration per se, we just work with what the tower wants to do.