With 24hr callout you could essentially be anywhere, if you are called you have 24hrs before whatever trip they assigned you to get back to base.

With Time Available, you are on duty or contactable during your regularly scheduled duty time for your trip. So if you were supposed to fly 4 hrs you are Time Available for that time and at the mercy of the schedulers.

I would take 24hr callout if you live out of base, but on the other hand if you had a short trip pulled Time Available may be more beneficial.