If you are not familiar with what these two are, these are drives. SCSI is considered to be more expensive than IDE especially when it comes to maintenance and support. SCSI can be expanded more though. It will be able to expand from 7 to 15 devices which are far more when compared to the number of devices that IDE can handle.
IDE will only be able to allow up to two devices for every available channel. When it comes to ease of use, IDE is considered to be better for beginners because setting it up can be a breeze. SCSI can be considered as a faster drive though which can make it worth the money you will spend on it.