I agree that the maker's skill is first order when looking at the quality of a certain design of whip.
But material matters, and for the bullwhip my dad developed, a design i think we are assuming you are considering looking at given your post here, kangaroo is generally better than calf. When the US embargoed kangaroo David worked in calf because he couldn't get kangaroo, the second he could get kangaroo, he moved to the better material and never looked back despite the costs and challenges in importing kangaroo versus sourcing calf locally.
Research tensile strength and scuff resistance of calf versus kangaroo, then look for a maker who would prefer to cut out a 450 series style bullwhip from calf as opposed to quality kangaroo leather... good luck!
You can get junk kangaroo skin, just like can get poor leather of any species. The tanning process really begins when the animal dies, you have to manage decay from the start with refrigeration and or pickling. If the shooter leaves the carcass in the back of their ute for days, then they've destroyed the unique qualities of kangaroo given by its tight cell structure:
http://packerleather.com/kangaroo-leather.html
Different whip designs are optimized for different leathers. Latigo makes a great rough and inexpensive four plait yard whip and if well tapered will crack just fine. There are many many beautifully optimized calf and cow leather whips out there. Cow and calf are relatively unsuitable for most finer work, well perhaps excepting very fine rawhide work, because of the courser grain, the tendency to lose strength on splitting, the directionality of the fibers, etc..
Getting back to basics, what do you want a whip for? If you want to put your whip in a display case or hang it on your belt for a Halloween party than the material doesn't matter so much, calf and kangaroo can look similar from a distance.
Your budget is of course a valid consideration as in all things like for example musical instruments: Sure I'd rather have a Fender american standard strat, they are far more playable in general than the mexican model i own. Both of these model's necks are computer milled, so the frets are in the right places, the workmanship is fine on the mexi. The cheaper wood, tuners etc... are mostly where the compromises are made, or not. It is not the best guitar to play, but for my second guitar, it was a wise choice.
If you are going to be cracking a whip in the wet, or on gravel, or if you don't want to worry if you lose it, a nylon or synthetic whip would be best. A well made good calf skin 450 series style whip may serve you very well for a long period of time given proper care, but, if you have the money, and there is a chance you are going to crack the whip repeatedly over decades and really want to hone your chops with the help of a fine instrument, you ought to consider ponying up for kangaroo in conjunction with your search for a quality whip maker.