buying new 02 sensor?

I go to both auto parts store and they are both saying there are 4 02 sensors on my 95 5.0..There is only 2 as it says in the book before the CAT.....But I went and looked at them and both of them are alot bigger then the one I took out. The one that I took out looks like the picture in the first link. Auto parts store looke like the second link. Even though I know the auto parts store is wrong should i get the 02 sensor that they say it for before the CAT or after that CAT.One is $65 and one is $45 so there must be some difference? They looked about the same but i know alot of cars put one right on the headers then one further back. The mustang is kinda in the middle...Thank!
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http://cgi.ebay.com/ebaymotors/SNG-...024QQitemZ370057456410QQrdZ1QQsspagenameZWDVW


http://cgi.ebay.com/ebaymotors/Bosc...025QQitemZ380033857603QQrdZ1QQsspagenameZWDVW