Selecting a tent

The differences in tents that make them suitable for a several week long camp, as opposed the an overnight camp are mostly related to how well it stands up to a driving rain and blowing mist. Any of these 5 tents will do just fine if you are car camping. If you get wet you can always go home or to a Laundromat to dry out your sleeping bags. If you are on an overnight camping trip, you are going home the next day anyway, so a small amount of leakage in bad weather is no problem. If you are out for an extended period, you have to not only stay dry, but actually be able to dry out your gear while it's still raining. Obviously, a sleeping bag is one of the more critical pieces of your equipment. If it gets wet it's very difficult to dry under field conditions. The 2 tents on the right are far superior to the ones on the left. They are variations of the same 2 tents. The ones on the right have a full coverage fly. The one on the middle left also has a vestibule.
Most people think of rain as coming approximately straight down. Any decent tent can handle that. When the rain is driven by wind, it can hit the sides of the tent harder than it hits the top. Any windows, zippers or openings of any kind are potential leaks. The full coverage fly takes the brunt of the rain and keeps it from reaching the tent itself. The fly is not in the way when it's rolled up, as in the top right and bottom photo, yet when zipped closed provides a substantial anount of protection as well as some gear storage.
The other soaking hazard is from blowing mist. It gets into everything. Hikers can find a sheltered spot, but paddlers will almost always be camped near water and in established campsites with little protection from the elements. The full coverage fly is also the best defense against the mist.
The vestibule gives you a way to get in and out of the tent in the rain without the rain getting in. What good is a tent that's totally rain proof only when it's zipped up tight? How do you get in if it's raining?
The tent in the upper left is suitable for over-night trips only. The front door and back window are exposed to the elements all the time. It cost $109 in Jan of 2001.
The one on the upper right is basically the same tent, with the addition of a full coverage fly. It's suitable for most weeklong trips, as long as you also have a lightweight tarp of some sort to cover your gear and cooking area. It costs $119 in Jan of 2001. That's only $10 more for a much better chance of enjoying your trip even if it rains.
The green one, on the left, is adequate for overnight trips. The large exposed end door and window in back are leakage hazards. It costs $99 in Jan of 2001. With the addition of 2 add on vestibules at $20 a piece. it's good for extended trips, but the total cost is up to $139.
The green one, on the right, has built in vestibules, which are more weather tight than the add-ons. It costs $119 in Jan of 2001 making it the best by of the 4.
For just a few dollars more you can buy a Kelty V2. It's this year's version of the Vortex 2, pictured below, that we bought for our Allagash trip. The big camping retailers usually sell off the last year's version of tents that have been improved. The 2000 version of the V2 costs only $129 in Jan of 2001. It has a substantial vestibule on both sides and a full coverage fly.
With only a $20 difference between the bare minimal over-nighter and a tent suitable for 2 weeks in rainy conditions, there is no reason to make do with less. Even if you can't afford a top quality tent made by Moss, Marmot or Walrus, a bit of shopping around will get you a more than adequate tent at a manageable price. Sure, plenty of people have used the lesser tents with good results and a little luck that the weather was good, but why take a chance on a miserable vacation when you don't have to?