Width – check the spesh BG site they guide you through picking a width. Remember to sit on the cardboard mimicking your bike position, the bones taper so a more upright position will require a wider saddle.
Cut out – as much to do with hip flexor flexibility as anything, tighter hip flexors mean a rotated pelvis which pushed the notcher down into the saddle. Big cut outs can be uncomfortable too.
Saddle shape – some roadies like their saddles to be a hammock to spread the pressure over the largest area possible, others like a scooped up tail as it provides something to push against, others like them flat to allow multiple comfortable positions. Typically MTB saddles are flat for this reason.
Basically a narrow Selle Italia SLR Gel Flow is the most comfortable saddle in the world for me and worth every penny, to you it could well be a £100 instrument of torture.