If I was looking a quicker turnaround time, I'd much rather have two sets of stairs than an airbridge, but that's considering it at my particular station.
For an A320 with full load, it's in most cases much quicker to disembark via two sets of steps and also board via two sets of steps, providing passengers use the right ones depending on their seat row. If you are parked remotely rather than just a terminal stand with no bridge, the availability of buses has to be considered. Provided there are two or three buses available, most of the outbound pax could be reconciled through the gate before the cabin is even ready for boarding. Thus allowing the bags of missing pax to be found early rather than having to wait until the queue of passengers filters down the jetbridge and onto the aircraft so you can process everyone through the gate.
So in reality it's probably a lot more dependent on the station in question and how efficient it is together with bus availablity. If it's your homebase at least you'd have an idea of their performance and be able to co-ordinate with them in advance if you know things are running late.