There are defintiely lots of great points for any of us to consider here. I'll also mention that like Tealtreak mentioned, I was always taught to be up an off the seat.... with a hardtail, you quite often have to aid the suspension that you have on the front of your bike... elbows and knees are always bent to help with that. And if you should ride like that, I can definietly see where it would take alot more energy in an endurance ride/race. Of course, we say the same thing with a fully, more weight ... more energy. The shorter travel full suspension bikes quite often fill that void, but as others have mentioned... it quite often can be decided with knowing the terrain you'll be racing. You mentioned the LT 100, that's a course where it's quite a bit of pavement, dirt roads and some rough... and it seems that people with both types of bikes will race that. Does your LBS have demos or bikes that you can rent? Thought that might be a way for you to 'ride' what you think you'd like to purchase... also find a shop that can truly 'dial' in the suspension for you and your weight that might aid in your decision as well. At any rate have fun looking for a new bike!!

PS I know from customers that I have sold bikes to over the years, the Cannondale Scalpel is a great short travel bike if that is what you are looking for.