In a word: Safety.

It usually costs a small company upwards of a quarter of a million dollars per year to comply with the 200+ laws that make up the federal safety regulations that the FDA enforces for medical devices.  The little Chinese companies making hand-held red light devices, and a few small US businesses that buy the Chinese stuff and re-sell it under their own names don't even obey a small fraction of those laws.  Whenever you are going to buy a device that claims to treat a health-related condition, it is wise to make sure the device is FDA-registered and the manufacturer is FDA-registered.  Unless both of those things are true, the device is being sold illegally.

Brands found FDA compliant

We had no problem verifying that the Quantum Rejuvenation red light therapy device is FDA-registered (see its current FDA registration here), and that its manufacturer (Holistic Technologies LLC) is FDA-registered as the manufacturer of the device (see its current FDA registration here).  We also had no problem verifying that the TendLite red light therapy device is FDA-registered (see its current FDA registration here), and that its manufacturer (Lumanosity Inc.) is FDA-registered as the manufacturer of the device (see its current FDA registration here).  

Brands found not FDA compliant

The above list of brands (marked with "do not enter" logos on the above list) lack FDA registration, and have numerous quality and safety violations.  They violate in many ways the FDA's more than 200 quality and safety regulations, so we do not recommend purchasing any of them.  We did test the non-FDA units before returning them, and they are all weaker than the FDA-registered units compared below, but we are not going to publish the numbers on the illegal units because they are safety risks.

As you can see from the above feature comparison table, the TendLite unit and the Quantum Rejuvenation unit are quite similar in a number of respects.  The key differences are in the last ​six lines of the comparison table.  ​Both units are FDA registered.  The key quality measure for this type of device is the brightness at the lens, which may be thought of as measuring treatment strength.

Although both units are so bright it is impossible to compare them with the naked eye, our lab tests show the Quantum Rejuvenation unit is 49% brighter than the TendLite unit.  That means the two one-minute doses with the Quantum Rejuvenation unit is equivalent to three doses with the TendLite unit, so you can get your treatments done faster.  It also means that the Quantum Rejuvenation unit treats to a deeper penetration depth than the TendLite unit, which means the Quantum Rejuvenation unit can treat deeper injuries the TendLite unit can't treat.

Some reviewers on Amazon who compared the two units said the Quantum Rejuvenation unit heats up more than the TendLite unit.  Our lab tests show that to be true if you apply the same amount of 1-minute doses, but if you compare equivalent amounts of light (say 6 doses with the TendLite unit to four doses with the Quantum Rejuvenation unit), they each heat up almost exactly the same amount.

Thus from the point of view of performance, and compliance with FDA safety regulations, the Quantum Rejuvenation unit wins.  It is also less expensive.  The only down side is that it doesn't come with a carry case.