I like both. OTK for the closeness and intimacy it affords of having the sub's penis between my thighs while I am spanking him. Over the bed is better for crops and paddles so that you can administer a harder spanking.
I'm not a domme (or a dominant), but I've topped quite a bit. You pretty much described my preferences already. When I want access to someone's body in a personal way, I prefer OTK or a position that puts the receiver in contact with my own body. Also, I position the person so I have access to... well... the parts I want access to! If I'm more in the mood for target practice or for administering sensations that are better created with a cane, paddle, or flogger, then I prefer having the person laying, mounted, suspended, etc. in such a way that I know they are safe and that provides "working room". Both kinds of play may be mixed in the same scene and sometimes, when I'm in a sadistic mood, I remember favorites... like small, rubber keychain floggers that look harmless, but are very effective at close range on certain body areas (such as breasts and inner thighs, or, if I'm feeling extremely nasty, the upper area of the buttocks below the lower back).
Side note 1: The upper buttock area cannot be hit hard as this is an unprotected, non-fleshy part of the body that can be easily damaged, but a little bit of sting here goes a long way. Little effort. Big return. Me like. Also, many people do not like this particular sensation (I'm one of them) so this is one of those "use carefully and sparingly for shock value" areas. Indeed, many people consider it an error to target this part of the body. For me, it falls under R.A.C.K. and under using appropriate care for the type of play and for the area of the body being targeted/stimulated. No, I'm not going to suggest aiming for the kidneys next!
Side note 2: Yes, this thread has been moved to the "Ask a Mistress" forum and it's a tad odd for a submissive to answer from the angle I have. Such is my mood today. :-)