My 2 cents. And it really is 2 cents, as I am not an SPL guy. The 'experts' here will have more to say on this, no doubt.
There is no 'best' answer to your general question. Usually you are trying to match the load to what the amp is optimally designed for and rated for - it is rare that an amp will put exactly same power into a 2 ohm load and a .5 ohm load, so your question as sort of moot.
Most amps will put more power into a lower impedance load, so as long as the amp is rated for it, many choose to use the lower impedance wiring configuration because they think more power = louder (and in many cases it is). There really isn't a point to having this discussion under your general assumptions. Ask the question again with a specific sub model and a specific amp model and you will get answers that matter. And the answers will change for different amp/sub combos. Thinking there is a 'correct' general answer to your question is dangerous and is why you see so many 'my amp is going into protect mode' or 'my subs are cutting out' questions on this forum.
The question of low impedance loads in very high power SPL set ups is how best to get the most power into the speaker's VC(s) (and hopefully into resulting SPL). This is a design tradeoff. At a given supply voltage, the amp theoretically can get more current into a lower impedance load. Since Power=V*I, that means more power. BUT, there is a limit, at very high currents, the internal series resistance of the amp's power components can cause excessive heating, so the amp must have good heat sinks and may need extra fans.
There are considerations on the VC side as well. You may find some dual VC subs that operate more efficiently (in terms of SPL per watt) in parallel versus series - or vice versa.
