I think regulator choice can be determined by sidemount style. If someone is diving sidemount configuration,meaning backmount type of diving but with tanks on their side,then generally any reg works. If you plan to really sidemount,which tank removal is probable,and the regulator can plow into the bottom,anything with an open ambient chamber is a poor choice. I don't use Scubapro MK20 etc for sidemounting,but prefer something that in environmentally sealed. In old school sidemount the Sherwoods used to be a regulator of choice,and is my preference.