I used the racial HP bonus for my game. I love the way they add to racial flavor, without giving too much to power-hungry players. It's just a bit better than the old 3.5 toughness feat - and I hate this feat, with its "mandatory for wizard" connotation.
Otherwise, a second best is the flat 6.
Basically, racial bonus or flat 6 are transforming an old 1st-level frail character into an almost 2nd-level. Enough to help survive, but not so much you do not feel threatened anymore by a puny goblin. Wizards will double their HP, warriors will just get about half as much - they still need one another to go explore this dungeon, so apparently no break of character motivation or class balance here.
Constitution score adding 12 or more, I'm afraid it will make low-level characters too resilient for their usual encounters. But some challenging games may require it. No issue with all of these options remaining options.
About racial HP bonus, I would put the Gnome race on the standard +6 list - in my view, their bonus to CON should cancel their malus to STR.