I prefer grilled to smoked for leg, but smoked to grilled for shoulder. All of it is good however.
For a leg that big, I'd de-bone and butterfly to try to get thickness as even as you can (no sweat--some will like more medium and some will like rare). This is what we normally serve for Easter (although I've had to do 2-3 legs and some chicken if 30-40 people were coming):
The night before marinade in:
6-8 big lemons worth of juice. 1/2 of them zested
6-10 tsp of kosher salt
about 1/2 as much coarse pepper as salt to as much pepper as salt
red pepper flakes to taste
1-2 tsp cumin
1 tsp coriander
1/4 to 1/2 tsp cinnamon
small bit of nutmeg
enough olive oil to almost submerge meat (this will seem like way too much but the over-abundance of oil and the lemon zest are the 2 most important parts)
After all of this is on the leg, put enough Italian seasoning or Greek herb seasoning to cover the surface entirely (this is the third most important point)
Add 1-1.5 large head of crushed garlic.
Again--this will seem like way too much oil and herbs, but Greek friends assure me this is correct
The morning of, get the lamb out of the marinade, put it up on a rack to dry in your refrigerator. Either strain out herbs, garlic, etc from the marinade or just use your hands. Rub solids on the meat and let dry in the refrigerator for a couple of hours (like while you go to church with the family) Save the oil.
After church we would light a medium grill (I like about 1/2 to 3/4 chimney of lump). While the lump is still hot, give each side a couple of minutes to get some color.
After that, cook covered. If you saved the oils, heat 1/2 of it up to make it safe to use as a baste.
After 15 minutes, baste with oil and turn the meat. Do this each 15 minutes and once as it comes off.
This has taken anywhere from 35 minutes to an hour of cooking depending on heat of grill, thickness of meat, and whether your family eats bloody or done lamb (I normally have to make one of each)
Be sure to tent and let it rest 20 minutes before serving
I usually cook to 120 internal which leaves some med rare and some medium bits of the lamb. 115 seems to have a lot more rareish parts but the edges will still be medium. 130 will be mostly medium with a decent bit of more well done parts.
Note--even though the meat is thick this should be done as written first. Given the flavor profile it is actually desirable to have some charred bits, some really rare bits and some medium to well bits. I tried a reverse sear once--while it was technically a better cooked piece of meat it really lost some of the atmosphere of the original
