I don't know.
I started with the cheap wand end with my search as a Commercial Vaillant Engineer was wafting one about that looked like a pen and I thought it looked easier than having a Gascoseeker swinging off my neck for checking leaks after working on Boilers. I always test around Gas valves, solenoids, joints etc that are only live when things are running.
Then I moved onto the dearer stuff like the one in shauns link because a Colleague of mine uses the Testo 316-2. It's great and it has three beep levels going from 10 ppm Nat Gas then 200 ppm then 10000 ppm.
I am pretty sure it also does Propane, Hydrogen as well.
It's £260-00 ish. It has no read out of levels though, just the noises.
So you know when you start thinking.
"for another £100, I could have that one"
On it goes. Hence the questions.
I know a lot depends on what kind of work you do. I do mainly Commercial gas work so I always tend to have a Gasco with me and that's why it gets so much use. It's not necessary to use kit like this all the time though.