I use the Fluval mentioned for several reasons.
Putting it inline lets me move the hazard of bad check valves away from the expensive parts water will damage. Needle valve, solenoid and reg are my things that I want to protect! I find gravity to be very dependable so I put the water down and expensive parts up higher.
Inline, I can put the bubble counter out where I can see it as well as fill it easy. The fluid or water does need to be refilled at times so I want it easy to do. With the Fluval, I can just unscrew the lid just enough to let a trickle of tank water flow back into the counter.
I also find it very cheap and easy to find them as many places do stock them. They are also plastic and they don't stand up to much abuse but then the expensive ones don't either.