I'm personally more tough than strong. Pain feels great, it tests my limits, and I only have room to be able to stand higher and higher doses of it. I love the challenge of it. Plus, it's not like I can tie myself up in ways I can't escape. 
I find being in charge boring personally, but a like... pseudo-mentor role can be pretty fun in it's own right, watching my partner improve as the sex and foreplay just gets better as a result. If I'm stuck thinking too much, it takes away from my ability to enjoy it. Being able to be the sub means I can just bask in it, let go, feel something, and apply very minor corrections to make it just right (or to have her retaliate from feeling challenged, which is even better).
There's also something... almost entrancing about being with someone who has the strength to hold the reigns. It's both comforting and extremely sexy to see such confidence. It's intoxicating in it's own right.
My partner and I are about equal strength in the upper body. She's a little stronger than me in the arms, and gravity certainly helps when wringing my wrists to the mattress. When she's in her more sadistic head space though I swear she doubles in strength.