Buying a better servo can give you any of the following depending on which you choose: faster speed, more torque, more durability, better centering, higher resolution (more sensitive to your input), 7.4V voltage capability
These are what I use:
http://hobbycity.com/hobbyking/store...idProduct=9983
These servos have amazing speed and torque. The alloy case and titanium gears make it very solid too. Awesome servo for the price.
The main downside is HK being in China means service and support will be very difficult, if available at all. You have to decide if the cheaper price is worth it to you. Buyer beware I guess. Some people don't like HK stuff and that's fine, but personally I have been very satisfied and never had any issues with these servos.