Each type of scanner has its own advantages and disadvantages. The flatbed scanner usually provides the best value for money. They have a great versatility too. Their disadvantage concerns the control. Film scanners are sophisticated, precise and give you lots of control but they are touchy. Drum scanners produce incredible quality but they are extremely expensive too. You should go for a resolution over 300 dpi and a dynamic range (Dmax) of at least 3. The scanning software is highly important too. Scanners usually come with their own software but you can buy a better third-party one too.
Save Money Using San Diego Glass Slide Scanning
Save Money Using San Diego Glass Slide Scanning
Source: http://www.lomography.com/magazine/tipster/2010/06/14/tutorial-all-about-how-to-properly-scan-from-equipment-to-technique