Do some research BigBlock.

LEDs have an extremely lightweight and compact design. This fact along with low heat emissions make LED's a very attractive light source. Also, they can be designed to emit the specific wavelengths required for photosynthesis (fluorescents and HIDs emit alot of wasted light).

BigBlock is right that LEDs are not cost efficient (yet), but there is a great deal of research and development in low cost production.

If you have problems with space and/or heat, and you have the money to drop, LED's might be for you.