I am interested in this too.
Great question and I'm sure you'll get a lot of different opinions on it. I've got a propane smoker but I rarely use it for chicken or turkey as it just doesn't get hot enough (325 degrees for me) to crisp the skin. There are a couple of things you can do to help fix that. 1) use butter with your rub. That's going to heat more quickly and give you that nice "mom just made turkey" color. 2) since you are just doing a breast you could place the turkey in a large cast iron skillet, breast facing down, once it hits 150 degrees. I'd put the skillet in the smoker prior to adding the bird to make sure it's already hot and doesn't slow down the cooking process. However, your idea of finishing in the oven (or grill) is a great solution as well. Just remember, if it starts getting too brown before the internal temp is ready simply cover with foil. It works like a charm.
As for the rub I personally like Cajun style rubs on turkey but have had good results with traditional rubs as well. I've used both of these and they are excellent.
Cajun (From Bon Appetite):
5 tablespoons kosher salt
2 tablespoons cayenne pepper
2 tablespoons garlic powder
2 tablespoons sweet paprika
1 tablespoon dried oregano
1 tablespoon dried thyme
1 tablespoon freshly ground black pepper
1 tablespoon onion powder
Traditional (From Martha Stuart):
6 Table Spoons unsalted butter, softened
Grated zest of 1 lemon
3 tablespoons finely chopped fresh flat-leaf parsley
3 tablespoons finely chopped fresh thyme leaves
3 teaspoons coarse salt, plus more for seasoning
1/4 teaspoons freshly ground pepper
On the later, I tend to loosen the skin and smear about half under the skin and the remainder on the outside. Then shove 3 to 4 lemons, quartered, and 2-3 small onions loosely inside the cavity.
As for using a brine, be sure to look closely at your bird. Most contain some % of a salt solution out of the box (self basting lol). Nothing wrong with that...but if you brine as well you could over do it with the salt. So you can 1) buy one without the solution (which is harder and harder to find) 2) try to remove it by soaking in water over night 3) use an injection to add flavor instead - which is what I've started doing. The key is to try and add the injection as evenly as possible. If not, you'll have areas with high concentrations of flavor and others with little to none.
Injection:
1 cup apple sauce or juice
2 teaspoons Garlic powder
2 teaspoons Onion powder
2 teaspoons Cajun Spice (such as Tony Chachere's Creole Seasoning)
2 teaspoons Powdered Mustard
2 teaspoons Chili Powder
Just my two cents. I'd be interested in hearing what others are doing.