I was first going to answer that technically you can't masturbate someone else, as the definition is 'self' stimulation. But dictionary.com says it can be another person as well; "stimulation other that coitus." So that would include oral as well... which doesn't make sense, but ...
Anyway, I enjoy both, but it's never that kind of a choice. Does anyone ever say 'gee I want to cum by using a hand. Now should I use mine or my partners?' Her playing with me or vice versa is a form of sex, so it's a very different situation, being alone and horny or being with your partner and playing with each other. Using your hands to stimulate your partner has always seemed very intimate and erotic to me, as you are literally holding someone's sexuality in your hands. Touching them and watching for reactions, listening to their change in breathing as they get aroused, doing everything to bring them pleasure.
That said I absolutely love to be touched and manually brought to orgasm. Only one LTR really learned how to drive me crazy with her hands, and I happily returned the favor! The best was on a Saturday morning as we woke up and I started lightly running my hands over her body. She was on her back and I'd start with her breasts and nipples before moving south and caressing her stomach and thighs. I'd be lying in my side and that was perfect positioning for her hand to wrap around me and squeeze and pull. I'd gently play with her pussy as she would stroke me to heaven. We could do this for over an hour, getting close, backing away, edging each other. She would stay continuously wet and drench the sheets and I'd release so much precum that I could literally fuck her hand like the tightest pussy or ass ever. We'd drive each other crazy and eventually I'd hear her slide over the edge and convulse in my arms and I'd watch her body shake and her chest heave while gasping for air. She'd squeeze me so tight when she came it felt wonderful.
A few times I would get a second orgasm out of her this way, especially if I was in the middle of whispering an erotic story in her ear as we were playing. After she came, I was absolutely ready to burst, so either she would finish me with her hand, or hand and mouth, or I'd roll her over and slide into her ass. No matter how we finished it was always a great start to the day!