Buying OBD-II Scanner

I want to purchase a OBD-II scanner but have been overwhelmed with the options availible. Between eBay and google there are way too many to pick from.
I am looking for reasonable bang for the buck, plain and simple. All my cars are GMs which I believe has VPW protocol and from what I am seeing a code reader is just that, a code reader. All seem to be the same in this aspect, they read the code.
Ultimately Id like one where I can load each of my cars factory set of parameters and be able to compare my cars true performance against those parameters. I have a laptop and dont mind using something basic like ELMScan if there are car spec data bases and fairly good software packages to go with it. Can you guys narrow the field for me and guide me to something ok for a backyard mechanic?
Also, Itd be really cool if it were backwards compatible with ODB-I???
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As for loading MFG specs and comparing I think you are expecting to much in a low cost scanner. Most of the advanced information is proprietary and you will have to go to the expensive MFG scanners to read it. I use the cable and software on my laptop from http://www.obd-2.com/#home and the grey matter between my ears to compare to the manufacturer specs. It works very good and the software is being upgraded regularly. It can be downloaded from the site.......

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