The square or concave trigger guard is intended for aiding the two-hand grip mode, and some trigger guards are grooved or checkered for the purpose of aiding two-handed hold with the off hand wrapped around the trigger guard. And it does hinder putting the gun back into a holster, and to a lesser extend is not helpful on smooth draw either.
For the opposite of the Square or concave trigger guard, look at the Mauser HSC, Whitney Lightning and Wolverine pistols, or go back to the Remington cartridge revolvers. They were designed that way for holstering ease.
I have an H&K USP with the square and checkered trigger guard. The scope mount that bolts to the underside of the frame in front of the trigger guard has a relief cut for the top finger gripping the trigger guard in two hand mode.
In one pistol model from the early 20th century, its concave trigger guard is actually attached to the slide and is designed to used by the trigger finger to pull the slide back for loading/unloading (don't want to use a two hand grip on that gun). (The trigger guard retracts the slide, but does not reciprocate in cycling: it works like the bolt handle of a UD42 or AR15/M16).