The technical specs vary from one unit to another one. Here are the technical specs that matter in usage and in image quality.

Radiation; kV

kV is the quality of the radiation. It determines the waveform and speed of the electrons flying from the tube head creating the main radiation beam.  Higher the kV is, the more greyscales the image has, and easier the software can form better image quality. All quality dental generator, both handheld and wall mounted devices have 70kV radiation.   

Radiation; mA

mA is the amount of radiation at any given time. Low mA means that the radiation has to stay ON longer to produce same amount of the radiation than counterparts than have higher mA. All quality wall mounted x-ray generators have 7mA radiation. Typical handheld x-ray generator have 2mA - 3mA radiation. This means that the radiation have to be ON longer. If the wall x-ray creates a good image in 0.08s exposure, that means that the handheld device needs exposure time of 0.18-0.28 seconds to create same image than wall mount x-ray provided that the kV in both units are the same.    

Radiation; Focal spot

Focal spot is the component inside the tubehead where the radiation originates. Smaller the focal spot is, sharper the images are. Typically focal spot size 0.4mm. Some cheaper unit have focal spot 0.8mm. Stay away from those units, you will never get a sharp image. Some newly designed units have 0.3mm focal spot. The difference in image quality is marginal, but still nice to have.   

Weight

Handheld x-rays weigh anywhere from 4.7 – 6.0 lbs. The lighter the unit, easier, especially for petite female operator, it is to handle and maneuver. 

Duty cycle
This one is a hugely important factor. Duty cycle means how quickly can you capture subsequent images. Duty cycle is mentioned in technical documents in a form of 1:30, which means how long you should wait before making next x-ray without overheating the tubehead. 1:30 means that you should wait 30x the used exposure time, 1:60 means 60x used exposure time. Some manufacturers give you time in seconds before next image is possible. This time WILL increase the more images you take. This time will start low, and when the device heats up, the time increases.  As an example,  you may be able to take few images in 3 second interval, but then the wait increase longer as the unit heats up. This is a nice feature if you need to capture few images (like a bitewing series) quickly, but do not expect to capture a full FMX  taking every x-ray in 5 second succession.

Cooling vs weight

Every manufacturer do a design compromises when designing the system. If you add more cooling capabilities to the device, the weight goes up and vice versa. Typically, manufacturers design a system that can take x-ray in every 10 seconds or so.