Looking at replacing my current bar. Slightly confused by the difference between these two measurements.

for example. a bar with 6º upsweep and a 10mm rise vs a bar with a 4º upsweep but a 15mm rise.

Would these not work out around the same?

Height may work out to be the same but the angle of your wrists will be different so may not feel the same.

^^^^ Wot he said.

Buy a bar with the width up/back sweep you like the feel of most.

Buy it in the rise that’s going to put your grips/controls closest to your prefered ride height

fine tune the height with headset spacers (or stem in extreme cases)

fine tune the sweep with bar roll (the angle you clamp the bars in the stem)