I've got HSGI tacos in both pistol and extended pistol and both work, although the extended give you more coverage unsurprisingly.
A lot of it can come down to play style and what other guns and rigs you run. You haven't mentioned if this is for belt or chest/plate carrier or both.
For example, HSGI do a taco that's rifle+pistol, so if you often run an M4 but also have occasional MP5 you can use it for either.
Alternatively if it's a more dedicated mp5 plate carrying rig, you might want all 4 to be quick draw across the chest (although I find this awkward with long mags and prefer a belt draw).
You generally don't *need* more than one or two of the tacos in my view. I have a couple on the belt since I tend to quick reload from those and then top them up from additional closed pouches on a chest rig in a lull.