Well, I’m smoking 15 pounds of brisket and 10 pounds of pork shoulder tomorrow, along with some hot links, so I think I’m qualified to help. 
(Actually, I will be starting in about four hours or so.)
First, pork loin. Being a lean cut, a brine really helps with this. If you’re going to do loin again, do yourself a favor and brine it in your favorite brine overnight (a basic 1 cup salt + 1 gallon water brine works fine. You can add a cup of sugar [brown if you like] if you want a sweeter brine. People are also fond of apple juice and putting all sorts of spices in their brine. Whatever you like. I’m fine with the bog-standard salt & water brine.) When you smoke it, pull it at 145-155 (you want it to be just a little bit pink. Pork is fine to eat at, IIRC, 137. I pull it at 150.) Wrap in foil and let rest for 15-30 minutes before slicing and serving. With the brine, you’ll get something somewhat like Canadian bacon as a final result. It is very yummy.
However, if you want proper pulled pork, you want to use pork shoulder (as you’ve noted). Bone-in Boston butt is the easier shoulder cut to deal with. (The picnic is the other side of the shoulder, or sometimes you can find a full shoulder, too.) I smoke my shoulder anywhere from 225-275. Usually, my smoker stays rock solid at 250, but if it’s really hot or really cold out, it can fluctuate ±25 degrees. I don’t really stress about the temp. 300 is a tad high, but you can do it at 300. At 250F, it takes from 6-9 hours to finish. Every shoulder is a little different.
If you can, do not use wood chips. Those burn way too fast, and wet wood makes for bitter smoke (from the creosote that is formed). Use wood chunks. They burn longer and cleaner. You DON’T want billowing white smoke. That’s the creosote-laden bitter smoke. You want thin wisps of bluish smoke. I advise against mesquite. Mesquite is better for grilling, but tends to be too bitter for smoking (although a little bit with brisket works okay.) Make a clean fire with your lump charcoal and add a few fist-sized chunks of oak (or apple) and some hickory or pecan for flavor. Some people are fine with straight hickory, I prefer the “sweeter” smoke flavors of oak and apple as the base.
Now, to help keep your temps down, you can put a roasting pan full of water near the outlet of your firebox into the cooking chamber. This will act as a sort of a heat sink and help keep the temperatures inside your cooking chamber more even and a little lower. Also, you can adjust your vents in your firebox to choke the airflow and keep the temp down. Always keep the chimney part of your smoker at least halfway open. You need the airflow to go from the fire box out the chimney. You do not want to choke your meat with the smoke.
Now, on to cooking. I start fat side up, cook for about three hours, flip, cook another three, flip and rotate, etc. until finished. Your pork is finished when it sort of “slumps” in on itself, looking like it can’t support its own weight. The bone should be loose, a fork stuck into it should offer little resistance and should be easy to twist. Temp will generally be in the 195-205 range when its pullable. At this point, wrap it in foil, let it rest for 30-60 minutes, then shred using two forks or with your hands (wearing kitchen gloves).
So, to let you know a little about what’s going on as you cook the meat:
Most of the smoke flavor happens in the first two to four hours of smoking. After that, your meat doesn’t really take on much more smoke. At a range of about 165-180 or so, the connective tissue in the meat begins to break down. Namely, the collagen turns into gelatin. If you are monitoring your temperatures, you will notice that there will come a point where the meat plateaus at a temperature in this range, maybe even drops a degree or two, and stays there for hours. This is when the transformation process occurs. The temperature will not continue going up until a good portion of this connective tissue has turned into gelatin.
So, once your temperature starts going up again, and you’re in the 185-190 range, you can stop here, if you’d like, rest the meat, and chop it. You will have chopped pork. This is actually my preference. However, most people seem to really like pulled pork, so you can do what JXJohns advises, and foil your meat, pack it in a cooler, and wait for several hours as the rest of the connective tissue dissolves. Or you can continue cooking it on the smoker. The advantage of cooking it on the smoker is you get a better bark. The foiling process, in essence, steams the meat so you lose a good bit of the crispness of the bark. On the other hand, a lot of people don’t care, and the foiling technique actually makes life a little easier on you.
As for rubs, I’m a pretty simple guy. I avoid sugar in my rubs, as they tend to burn and get a little bitter. I do it differently every time, but I start with salt and pepper, and I tend to like the “sweeter” spices like allspice, clove, even fennel, in my rub, along with the more common mustard seed, celery seed, garlic powder, onion powder, paprika, etc.
Any questions?
(And, of course, any discussion of barbecue will lead to a multitude of differing opinions. That’s part of the fun.)
