Just my usual comment: neither. Stiffness and muscle weakness are the usual causes of back pain while cycling. Stiffness, because it doesn't allow you to assume a pain-free position with hips rolled forward and straight back. Weakness, because cycling involves the whole posterior chain which has its upper end just below the shoulder blades. There's a good book about cycling and core strength: Core Advantage.

For some reason, I think vanity, ab exercise has become what people think of as "core work." Wrong. Back work 5X what you do for ab work.

All that said, IME a more stretched out position is less likely to produce back pain. Given a sound and strong rider, bar height doesn't matter all that much. Extremes seem more likely to cause problems: getting close to a horizontal back and back angles of less than 45° seem more problematic.