For me,
- Pricing = I can't find any good budget LED
- Ease of use = 50/50
- Coral health / growth
- Overall outlook of the reef (colours pleasing to your eye?)
- Design (looks like a freakin' cool toy)
- Power consumption
- Customizability (modular system, interchangable parts, etc)
- Upgradability
- Wireless control
- Syncronization with 3rd party equipment (ie. app control through smart phones, controllers, etc)
- Touch panel control
- Dimming / in-built timers
- Special features (ie. lightning, cloudy day simulations)
- Accessories
- Build quality
- After-sales support
- Hybrid options (coupled with T5s)