The Combro CB625 is around for a long time and a trusty and proven chrono for 12 fpe rifles. I have used mine for more than 10 years now. The first one I had did not survive prolonged testing of a FAC airrifle (>40fpe). I did not really break down but the sensors started moving out of their intended place and this caused constant error readings.
The Xcortech is designed for airsoft and I have read a quite a few negative reviews when used for real airguns. Seems not to be very accurate and may give false readings.
The Bluechron is a bit like a newer type Combro with a color display. I have read one positive review. It's made out of alloy where the Combro is completley made of plastic. Has to prove itself yet.