the question you need to ask tenkan is to turn in circle, where is the center of the circle? you or uke? which is better, you move around uke or uke moves around you?
In a martial sense, the situation dictates where the center is located. You should be good at all three: you are center, attacker is center, mid-point between you and attacker is center.
Between environment, obstacles, buddies of the attacker, and weapons, your choices of movement may be limited. You may have to move the attacker around you, you may have to move yourself around the attacker, or move both you and the attacker.
None of them are "better" as all three should be trained, along with disengaging and re-engaging, and vertical levels. In the aikido area, this is why *good* randori training is necessary. You train engaging someone, disengaging if a more imminent threat is present, re-engaging, various center points in pivots, and using mid-vertical level (going to one knee).
Of course, there should be a lot more being trained ... which brings us back to the original topic, tenkan.
Backing out, moving backwards, stepping out of a tenkan are all common issues that create problems. To try to correct that, turn the spine first. Think of the spine as being free in your body.
1. Imagine stretching the spine upwards and downwards and that it's a straight line. Now imagine the spine turning (clockwise or counterclockwise) in place.
2. Once you have that, start turning the upper body (from both shoulders all the way down to the "V" area between the hips) around the spine. Don't curve or kink the spine.
3. Then at a point where the upper body turn starts pulling the hips, complete the turn. Don't move the hips first or second.