1) Arb clock is determined by service time on the major league roster. So you'd be looking at around 27-30 days. Minimal impact.
2) Making the ML playoffs should trump all other considerations. If they can help you at the major league level, call them up. But if they're not going to have much impact at the ML level, and will mostly be sitting on the bench, they might be better off getting the extra playing time in AAA.
The question you didn't ask: adding them to the 40 will start their option clock, so they'll start burning through their three option years.