The best lamp to use is the one that goes with the system you are using. If its Shellac then use the CND lamp, if its Gelish, use the Gelish Lamp, if its GELeration, use the GELeration lamp.
You then know 100% that the lamp will be completely curing the product it is designed for. Just because it emits the right type of light doesn't mean its the exact same light. There are so many factors that come into play in these matters.
Also if you are offering a Shellac service but use a lamp that isn't certified / approved for use with Shellac then you are not offering Shellac. To offer a the named treatment you have to use and follow the products, procedure and standard set out by the manufacturer! To do anything different would be misleading to your clients, and potentially damaging to your reputation.
LED technology and lamps have come on leaps and bounds in the past few years, and they offer an exciting alternative. However that alternative comes at a price ... two to three times the cost of a UV lamp. You need to be very sure your business can justify that cost, and over how many treatments you will recoup it.
Mat