Initial thoughts
Appearance is subjective but i prefer the look and feel of the viper. Particularly dislike the screw cover on the impaq. Honeywell Viper comes with a manual the impaq does not. Like the built in resistors on the impaq, prefer the cable room on the viper.
Included in the box
Texecom Impaq S items in the box 
honeywell Viper GL items in the box
Installation and operation
On the Impaq you need to remove the pcb to get to the mounting screw location. not a huge issue but something id prefer not to do. Obviously you would remove the viper pcb but at least its a simple operation. However I feel the inclusion of the correct size screws in the viper a massive plus over none in the impaq s
EOL Inclusion. I like the selectable resistors on the Impaq. Whilst the resistors required are included with the control panel i prefer built in resistors every time. But there seems no way to use the impaq in a double pole system. It looks EOL only so make sure your panel is compatible first.
Both units have comfort LED functionality. Both units flash every few seconds to show they are powered, showing a different indication in alarm. This can be disabled
Both devices have adjustable sensitivity however they are very different. The Viper employs a variable adjustment providing hundreds of settings. The Impaq uses a button system allowing 5 levels of adjustment. While this ‘will do’ I prefer the viper approach as it allows for much greater control.
Both units were reliable in operation. But reliability would depend on the correct setup.
as a final note i found this in the texecom manual which i have never seen before
Domestic Use: If this product is installed within reach of children the screw cover should not be fitted as this could cause a choking hazard.
Conclusion
Both units do a fine job of detection, both are EN50131 compliant. There isnt any major minus points on either device but due to the adjustment, quicker installation and cable entry/space I prefer the Viper unit.
