OK,
so the first time I posted I hadn't actually gone and looked at what the records are...
but now I can say it looks like this
stealers
super bowl wins
Six (75, 76, 79, 80, 06, 09)
super bowl losses
one (96)
49ers
superbowl wins
Five (82, 85, 89, 90, 95)
superbowl losses
(none)
so the stealers have a 6:7 (85%) success rate (at this one game)
49ers have a 5:5 (100%) success rate (at this one game)
If you look at the past as a good measure of the future,
if the stealers make it to the superbowl next year they have a 15% chance of loosing,
if the 49ers make it to the superbowl, historically speaking they have a 0% chance of loosing.
thus the 49ers have a better advantage psychologically.
if you say that the stealers are better as they've made it to superbowl more times, then that would also suggest that the dallas cowboys are better as they've made it 8 times, (though they've lost it 3 times 62% success ratio)
or you could say that the Stealers have won it six times, though that's only one more than the 49ers.
when you look at chances of getting to the superbowl there have been 43 superbowls,
Stealers have been there 7:43 (16%)
49ers have been there 5:43 (11%)
so stealers have an edge there, but the percentage gap is smaller, (with only 4% difference you can't really base a statistically significant psychological advantage like you can with a 15% difference in win ratio...
Of course, that's only a psychological advantage, and stats are only good if you're making top-trump cards.
the real advantage would be shown in how each team performed in the season before hand. and that would give a more significant psychological advantage)