Hello there.
For guild missions we get the options: PvE only, PvP only, WvW only and all 3 types with +10% to favor.
I assume there are a lot of guilds who would enjoy an 2 gamemode option.
My guild does not enjoy WvW, but all of us like to participate in the weekly PvE missions, and most of us play sPvP and we enjoyed the PvP guild missions last week.
To me no +10% or +5% to favor would be needed, just an option to check 2 out of the 3 gamemodes.