I've done both. Here's what I've learned in 40 years of cooking turkeys.
1. Breast up or down makes no difference in juiciness contrary to some of the advice out there. Brining is about the only way you can impact the juiciness of the breast.
2. Whatever is on top will generally cook a little faster.
3. The bone density in the thighs means they'll cook slower.
4. Using a turkey rack, turkey cannon, vertical rack, or beer-canning a turkey tends to even out the temp differences between the breast and thighs, but not eliminate them.
5. If the bottom of the turkey is shielded from direct heat because you are smoking or roasting it in a pan filled with veggies, whatever is on bottom (breast or thighs) will cook even slower than the top.
I use a turkey cannon and put the drip pan filled with seasoned veggies and some cheap white wine on the grate below the turkey to catch the drippings, roast the veggies, and flavor the drippings even more.
I smoke, roast, or smoke/roast breast side up.