i think it's both, but in my own experience = matters less when sitting. i don't have any axes that break 8 lbs., but if I did i'd be more inclined to sit --- just for the groove.....
(an emoji of a sitting player...playing an unlined, fretless, 4-string PJ.)
+1 on this. Weight can matter, but I believe how the instrument balances across the knee matters more. Does it sit well naturally, or do you have to devote some attention to keeping it in place? How far away is the nut? Stuff like that. Although those big ol' Gretsches present a lot of ergonomic challenges when played from a strap standing, they are among the most comfortable basses I have encountered for playing seated. Even Bassoguitars (and 1/4 size DBs) can be played comfortably while seated.