These units all basically do the same thing. Unless you have all of the technical gear and equipment to check and adjust engine RPM's, torque things like that, the higher end units that run off a laptop are overkill.
We own three Audi's, and have used the Equus 3100 Innova Diagnostic Code Reader with Memory Backup for OBDII (Post-1996) Vehicles
Amazon.com: Equus 3100 Innova Diagnostic Code Reader with Memory Backup for OBDII (Post-1996) Vehicles: Automotive. I know most of the auto stores have this unit. Costs about $100.
We use it on all our vehicles (and for a few friends too!!!). Whenever the "check engine" light comes on, we hook it up and get the code in less than 30 seconds. Could not be easier.
There are many "ghost" codes that are very difficult and expensive to rack down with certainty. One of the best uses is to check the code, find out what it says in wrong, turn off the code, and let it run for a while to see if it comes back on.
Once you get the codes, you can go to a user forum on the web and get very specific and detailed info on what can or could be done about the problem/issue.
For example, we have a torque sensor possibly bad on one of our Audi's. The repair involves taking out the transmission and would cost about $1,500 possibly more. One mechanic online suggested just unplugging the sensor, noting that it would not affect performance, just lose a few miles per gallon mileage. At least it's an alternative --- one that no dealer is going to suggest.
With various kinds of emission devices, changes in altitude, gas mixture and other factors can set off the "check engine light." Today most dealers want $50 or even $100 just top "read" the codes. Believe it or not, I even heard a local dealer tried to tell someone it was "illegal" for car owners to use code readers. That was the laugh of the year.

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This unit we have has worked perfectly for almost four years. We have used it to both analyze problems, and also avoid a couple of very expensive repairs that were advised, but not really necessary, because the codes never came back on in three years after they triggered once.
I cannot give these devices high enough praise. Every vehicle owner today should own one, period.