I think cardio plus resistance exercise is best. Whatever might be going on with my lean mass, I definitely feel better if I get in at least a couple of miles of walking per day. Endurance exercise also does preserve lean mass, it's just that most people have already gotten all the muscle-preserving activity into their day to have already gotten all the benefit they're going to get from that level of resistance.
One problem with endurance training for lean mass preservation during weight loss, I guess, is that the weight used for the exercise is by definition going down, this deloading during normal day to day activity might be part of the reason that people lose lean mass when they lose weight in the first place.
If you look at a one rep maximum calculator like this one,
http://www.bodybuilding.com/fun/other7.htm
if you take an exercise where you can do 10 reps at 80 pounds, they calculate that you can only do 1 rep at 102 pounds. If you can do 5 reps at 90 pounds, that's 1 rep at 98 pounds. So you could see how weight loss could really reduce the lean mass protective effect of incidental squatting in and out of chairs and stair climbing etc. in normal day to day activity, relatively small changes in weight used translates to massive differences in intensity.
One other thing--weight training can be intimidating, but the prescription doesn't have to be. There are a number of studies showing that even at 30 percent of one rep max, there's a good anabolic effect from weight lifting, comparable to the effect of training at 80 percent of one rep max. That's something like 25-30 reps to failure, compared to say 8-10. The most important factor seems to be that the exercise needs to be done to failure. I use both rep ranges. One advantage I like about the higher rep range, is that the earlier part of the exercise pretty much counts as a warm-up. Also, even upping the rep range just a little bit gives you a better pump, that helps to stabilize and protect the joints. Another advantage is that it's easier to do some exercises at home at the higher rep range. If I fail during squats at a 10 rep range with a barbell while alone, that's pretty risky. With a 20 plus rep range, I can do squats holding dumbells at my sides and just let go when I'm done.