Bioth are effective, depending on the situation. For example if you go for the armbar from side or knee on belly you end up in the bent-resting-leg variation. Trying to pass that leg over makes no sense and puts you in danger of your opponent's escape. As long As you keep the head down and keep your knees together you should do fine.
The armbar; is it more effective/ proper to have one leg over his head and the other bent resting in his side or both over his body?
I don't quite agree. Marcelo Garcia throws the leg over the body on the far side armbar from side/KOB, and teaches his students to do the same. You have more control over the body to stop the hitchhiker escape with the leg over the body. It also stops the back-roll escape. The knee-in-the-armpit variant gives you a nice option of shooting the inverted triangle, though.
Depends on your goals or strengths.
Both legs over: you can cross your feet (as long as your feet are crossed under his armpit/behind his shoulder) which will stop the running man/hitchhiker escape. Also allows you to sneak that bottom leg through his arms if he is hanging on for dear life, going right into a mean triangle. Also allows you to switch to bicep slicers, also allows you to abandon the side armbar, go to a tight S-mount, and then trap him into giving you the opposite side armbar.
Bent leg: For most people, it's faster to enter this armbar from side control (far side spinning armbar as well as near side his-hand-under-your-armpit armbar). As someone mentioned, you risk a fast and smart guy getting to his hitchhiker escape, BUT can counter by slipping the bent leg behind his body and into a reverse triangle a la Braulio.
Matter of preference then I suppose. I feel more secure with both legs over, although I am not always in a position to get it.
How? My way was sort of a wind-shield wiper.I pressed my knee on top of the armpit and slid the leg against the opponent chest.