I've done both these turnarounds. CGK is quicker (by a few hours), but have hassle with departure tax (pay at gate desk in rupiahs, S$ or US$ - no change given). Lounge at CGK is IMHO better than the SQ lounge at BKK, although not as good as TG lounge at BKK. Get maybe 20 minutes in the lounge at CGK but a lot more at BKK.

Only BKK has 744 flight (currently one per day each direction) if wanting sky suite. Only CGK has A345 flight (currently 2 per day each direction) if want more room.

For me the schedule also matters (particularly mid afternoon/evening).