Bicycle Mechanics - Derailleurs, What makes them better?

Im sorry if I sound stupid asking this,I really dont know. What makes one Derailleur better than another. besides weight. On my MTN bike I have a Deore Derailleurs on front and back, if I was to upgrade lets say to a Deore XT what difference would it make, same thing with my road bike, on that one I now have front and rear Tiagra, if I was to go to the 105 or Ultegra series, what is going to change

They'll be lighter and cooler looking. Once you get into the Ultegra-level ones, the pulleys have smove bearings, too.

My feeling is, if someone has to ask, the answer is probably to go for the most durable — and probably pretty cheap — one. A malfunctioning derailleur is a drag, and the weight savings aren't worth having to replace it because you leaned it against a wall. And if you screw up a derailleur that's cheap, it's cheaper to replace, too.

I guess what I am really asking is what is the difference in how they perform, meaning do they shift better,faster? I am not really worried about cost. But i have noticed that cheap ones are 30-40 and higher end run close to 200 or more thats really a big difference in price.

Higher end derailleurs will have more precise movement leading to more accurate shifting. This 'benefit' depends on keeping everything squeaky clean and well lubed all the time. It's not going to take much dirt buildup to eliminate any advantage. For most people, it's not worth the extra bucks. bk

For the casual rider, Deore or 105 grouppo and up yer fine. Above that is for people who perform periodical maintenance due to high mileage and the components need to be disassemble/re-assemble periodically and able to take it.

Or if u just feel like it or like the blink.