I'll weigh in here (ar ar) . . .
IMHO both are important. Heavy weights are necessary for absolute strength development, which is quite important because the more and heavier you CAN lift the more you WILL lift. Also, the heavier you can / do lift the more your skeletal system will benefit, because your bones will respond to the heavier weight loads by becoming and/or staying stronger and denser, no small issue as the body matures. If you want to minimize the need for pharmaceutical intervention for bone density loss prevention, heavy lifting coupled with a healthy diet is a must. Also, absolute strength development is critical for daily functioning, and yes it promotes mass development better than endurance training.
HOWEVER . . .
Endurance training is also important, because by definition it means you can go longer at a submaximal load, and that more closely mimics life's daily demands. In daily functioning you're not really asked to go the single-rep max for a brief period of time. You are instead asked to go for a longer period of time at a lower resistance (say, doing a ton of house and yard work). Targeted endurance training can promote muscle MAINTENANCE, both in muscle performance and muscle mass.
I believe I recall reading years ago in one of my ACE publications that absolute strength development assists in muscle endurance training; however, the reverse was not true (i.e. endurance training did not really promote absolute strength development derived from lifting close to the heaviest loads you can handle). That being said, I still believe both are crucial.
Tallchick, I personally think there's a protocol still out there that will enable you to build muscle mass AND absolute strength. If your screen name is any indication, I'll wager you're taller (most are taller than The Bald One, who tops out at 5'3") with longer limbs, and that puts you at a bit of a mechanical disadvantage when working to build mass (longer levers have that effect). IMHO, the Slow And Heavy series would be a good one for you to adopt on a regular basis. Also, I'd like to suggest that you "steal" 2-3 sets from your favorite Cathe strength routines that hit all of the upper and lower body muscle groups, write down a routine of these, and do these on your own, with the heaviest weights you can handle to a maximum of 12 - 14 reps, and do them at your very own rep speed. And get little gizmos that enable you to incrementally increase weight loads, and set goals for increasing these weight loads every 6-8 weeks. Plate Mates are good (GREAT, actually) to attach to metal hex dumbbells, and there are little 1.25 iron platelets you can buy online or at emporiums like Sports Authority to put on your barbell and incrementally increase your barbell load that way. Also, if you're confined to home workouts with no gym squat rack, consider investing in a weight vest that enables you to put in more and more iron rods (they come in 1/2-lb increments) as your legs get stronger. Allpro makes a good weight vest design and sells two versions, one with a max weight load of 20 lbs and another with a max weight load of 40 lbs (I have the 40-lb'er); there are others.
JMHO -
A-Jock