Totally up to you and what you're looking for. There seems to be some divergence from what Wikipedia lists as part of the War of Kings and Realm of Kings events, and what's actually collected in the omnis/OHCs. Right now, the GotG omni is listed as containing issues 1-25, plus The Thanos Imperative: Ignition, #1-6, and Devastation (the complete miniseries, basically), and the Rocket Raccoon/Groot backups from the two Annihilators minis. The War Of Kings omni is set to collect GotG #13-19 (I guess issues 7-12 are considered prologue? I really don't know), and the Realm of Kings OHC appears to not collect any GotG issues.
So if you get both, it appears that you're only double-dipping on GotG #13-19. GotG #7-12 is listed on Wikipedia as part of War of Kings, and issues 20-24 as part of Realm of Kings, but those issues don't appear to be in the WoK omnibus or the RoK OHC. So maybe they're ancillary to the main story, I really couldn't say.