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.
If you're lucky enough to be on private land and can leave your stand up during the off season you can tie an old piece of cloth or something of the sort to your stand. I like to use an old camo bandana that I have. That way the deer get used to the movement and eventually don't bother even looking up when there is slight movement. This may just give you the time to draw your bow or raise your gun without being detected.