I always think of running as THE aerobic excersize, I mean you also never know when you might in a moments notice be able to get the hell out of somewhere in a hurry. More importantly you'd look just plain ridiculous skipping down the street if you're in the double digits for age...
skipping rope can be very intense as a cardio workout however much like other cardio exercises you must have a high effort or it becomes more of a timing exercise versus a heart pumping cardio workout.
SKIPPING IS AWESOME IM 15 I LIVE IN NEW ZEALAND AN I WEIGH 160 POUNDS(73KG) AND I SAW A DIFFERENCE IM MY CHEST AND SHOULDERS AND ABDOMINALS AFTER 3 DAYS WITH A SKIPPING ROPE AND NO OTHER EXERSIZE!!!@!!!AFTER A WEEK I LOST TWO POUNDS FOR ONLY 10 MINS WITH A LEATHER ROPE A DAY !CHEERS
Why not do both? Skipping brings in the upper body a little more but there are no hills while skipping. If you only run on flat ground ie. A track, I would guess that skipping, at the same intensity as the run, would burn more cals. as the upper body is more involved. Running when hills are involved really gets the legs and cardio going.