It ends up being a personal decision based on your needs and preferences.
That covers it all right there.
I prefer the oem cases. Advantages:
Streamlined to the bike, lightweight, integrated bracketry
Takes up less garage space, easier to park
The convenience of 1 key for everything on the bike-ignition, seat, gas cap, and luggage
Ive never had things fall out of my bags when opened--they have internal retaining straps
The dealer left the side cases and brackets on by mistake and didnt charge me for them(!)
With the top case on the bike, Ive learned to be a contortionist getting on and off the bike
They feel a bit fragile
Stupidly expensive retail
Repair parts not available separately from Suzuki-break a side case or any part of it, buy a case set
The right side case is smaller inside as it loses space wrapping around the muffler
I ordered a Suzuki oem accessories tailbag made for the '12-16 Vstrom650 from England(not sold here in the US). It fits the '14-'18 Vstrom1000 perfectly with the top case mounting pad. It's a Bags- Connection piece, matches my oem Suzi tank bags. Smaller than my top box, easier to get on and off the bike. The side cases and rear bag are always on the bike unless I need more room for a trip, then the rear bag gets exchanged for the top box.
Forget putting a helmet in the side cases, too narrow. My old helmet fit in the 35L top box, my new helmet does not. But Suzuki now offers a 55L top box, supposed to hold 2 full-face helmets and whatever else. But...it requires a metal mounting bracket for the plastic rear rack, is approx $950(!!!) retail...and is not supposed to be used with the side cases installed.
For my purposes (and what I didnt have to pay for) I'm happy with my oem cases. Your needs and preferences may differ from mine.