"While an open lander is possible, I predict it will not happen soon. If you expect to stay at all, you need a habitat, and at least initially, it wouldn't be permanent, so why not make the lander the hab?. Later if we really have the resources to support a base we probably can support a pressurized lander, most likely even larger than the LM. I predict The next will be 3 to 4 times the payload capacity of the original, with conversion to cryogenic fuels when and if ice deposits can be utilized. Large boosters can be derived from STS and Delta IV hardware for a reasonable development cost."

I would say if you only wanted to stay on lunar surface for several days and weren't planning on the crew doing very much, it makes sense to design lander so that crew can live in it.
But if you want a larger living area and lot's of supplies. And don't want expense of building a large booster or building in orbit a larger vehicle with multiple launches, then you could look at the option of pre-landing supplies at a base. Pre-landing supplies gives you a lot of advantages but with one main disadvantage: You have to get the crew to the area you have pre-landed your supplies. This disadvantage can be mitigated by other factors you can design into your mission- and things you do could have other advantages which taken as a whole could reduce risk to crew, increase their capablility on the surface, and lower overall costs.
With prelanding you can land your return vehicle, your base, supplies for life support, and equipment and materials needed for mission, a beacon for crew landing, power supply, and communication to Earth.
The crew lander just lands crew- everything else is stripped down. There are various options you take to make this vehicle. First it could be a staged vehicle- this would be not refuelable- at least the first stage.
The first stage handles most of the delta-v needed for descent, and second stage could land crew on the surface. So a mile or so from landing area the first stage could be jetisonned- it crashes on lunar surface, and it's simply a rocket stage. The second stage has crew and landing gear. The crew could be standing and be in space suits- open cockpit. This second could have "flying range" of 10 miles on the lunar surface and be refuelable, by which I mean, it could take-off, fly 10 miles land, then return to base. Now, it could be designed for flying range of 5 miles, 20, 50 or 100 miles, or have one vehicle capable of doing all the above, by adding tanks. And of course additional tanks of rocket fuel could be pre-landed. Now, you might want to first land it with the 100 mile range. Some of the tanks could be removed so that when on surface this "extra tank space" could be use to carry equip and/or samples/cargo with limited range.

In order to keep the post short, I'll end it there.
-gb