It's a huge difference.
For starters dated 3 watt LEDs produce a lot more heat. This heat reduces efficiency and requires better thermal management.
EBay Fixtures use these LEDs with little thermal management and the result is a disposable fixture. One that cost the same as a MH or T5 bulb replacement over 2 years.
Higher end fixtures generally use Cree LEDs. These are binned in 5 intensity variations and 28 different colour variations for the cool white.
Crees X series also has intensity binning with the brighter bins being more efficient producing less heat and more light.
Here is a pic of two different Cree cool whites. One is a warmer bin the other is a cooler bin
Unless you specify a Bin, LEDs "are like a box of chocolates" You dont know what you're going to get.
Its almost impossible to verify the Bins of Epistar and Bridgelux LEDs from suppliers.
Most companies do not specify bins as its more expensive to choose the brighter R5 and the cooler temps like 7.5K are less common than the 6K.
The R5 Bins can be 20% brighter than the R1 or R2 using the same electricity.
I always looked at creating a LED fixture like a Chili Cook Off.
Some are worthy while most simply do not measure up....Its all in the ingredients...
There is a lot more information online.
Hope this helps
Bill