Modern 'C-Shape' booms allow the wrist to remain straight (see diagram), which means less stress on the forearms.
For some this will mean longer, funner sessions on the water, because the power can be controlled more efficiently with the 'straight-on' stance they facilitate.
C-Shape booms' increased parallel-tube outlines also allow the back hand to be nearer the body and therefore the body to be more committed when bottom turning for increased control both in wave riding and in the air.
Of course the outline also means the parallel tubes are under less stress and so the C-Shape is also markedly stronger.
Regular, or 'R-Shapes' are the traditional, classic, ‘wishbones' with an outline shape that fits the curvature of a cambered sail.
R-shapes place the front hand closer to the luff, which is often more comfortable for those that favour an underhand grip on their forward side.
Which is best? Well, racers often prefer the R-shape, especially in larger sizes of say more than 200 cm. and/or sails over 7.0 sqm. Slalom and speed sailors often also favour these outlines.
The C-Shape is mostly suitable for free riding and more dynamic disciplines such as wave sailing or freestyle.