East of I-25 Whitetails more than likely number close to mulies. Whitetails will be thick along the river bottoms. Mulies like the sage flats and mountains more. West of I-25 would be better mulie habitat. Whitetails are really spreading out and slowly taking over.
Whitetail populations are really growing now west of the Bighorn Mountains all through the Bighorn Basin and mostly in the lower river valleys and drainages like Hiker mentioned. If you will look in the Wyoming G&F online hunting booklet for 2011 you can see where there are extra whitetail tags and that will give you a pretty decent idea where the bigger numbers are.
The mule deer population of Wyoming is estimated to be between 450,000 and 500,000, which is down from the 600,000 that was maintained for many years.
Finding numbers for the whitetail population is much more difficult. By adding the estimated populations of several areas where they are found, I came up with approximately 65,000. The largest population is found in the Casper region and the next highest in the Sheridan Region.
Here's a map that may be helpful in your planning:
In the Northeast Region, Region A, they outnumber mule deer 2:1. Most of the other areas mulies outnumber whitetails 5:1 to 10:1, but of course that's averaged over the whole area. Whitetails tend to outnumber mule deer along the river bottoms, but in some places it's about even.
Bring them in on a string. I love this term, if you want to be successful on calling in Elk, try not to just sound like the other caller. We all laugh at each other when we are practicing Elk calling at home or while camping. When I first started calling in Elk, I had no idea what i was doing and it's funny how many I could call in. The biggest Bull that I have called in to date was actually my first. I was making cow calls and screwing it up big time. But it's obvious I didn't screw up...