Well, i would say it depends really. Especially in europe at the moment the only low cost carrier that is all boeing is Ryanair. Easyjet, Wizzair, Eurowings, Vueling, Level are all pure airbus operators. Of course, with the newest order for 200 737 MAX from IAG that will change somewhat. But even in the US the picture is a bit more complex. Sure, Southwest is a boeing operator, but Frontier, Spirit, JetBlue are airbus operators.
All in all airbus and boeing are pretty much interchangeable for low cost ops. Yes, for the 737 you do not need in all cases a conveyor belt to load the luggage, in an airbus you pretty much do, except if you go for container loading (which low costs in my experience do not) which of course needs the required equipment for that. That said, 20 minute turnarounds are possible in both types, in my view the airbus actually has some advantages as it can fuel on one side and load on the other side without any interference (fuelling ports are available on both wings). Of course, in the case of left side fuelling passenger handling might be a problem. I haven't seen an A320 with a built-in airstair, although i head that it is a customer option. On the other hand, i have first hand experience with a built in airstair in the 737, and ryanair for example has all its 737 equipped with that, that airstair helps with handling, but of course has a 200 (or was it 250?) kgs weight penalty that is always carried around. In the end, the beancounters will decide what they do. And surprisingly even Ryanair now has an Airbus fleet with Laudamotion.