Once you start reading you will find that there are FEW true differences between Impala, 9C1 or the civilian car which get far too little respect. All had the exact same engine and transmission, same power ratings, the tires size differences make as much of a difference as the minor gearing differences and they all have sufficient cooling systems etc. If you are looking for something where you are willing to spend a lot more keep is stockish and retain better resale, or if image is important to you then absolutely the Impala is the way to go. If you are looking for something to drive or mod the hell out of then consider a civilian car or a 9C1. Yes each had different spring and shock setups but the shocks are long since gone and at 10 years old springs are going to be well worn and replacement not a bad idea even if you want to keep stock height.
People want to believe the Impala was a big performance package, it was primarily appearance. The true performance enhancements were 154mph speed limiter the civi was 108 the 9C1 had a few option up to 147 even with enough road stock car usually wont quite make these numbers. The 9C1 and SS both shared 3.08 and rear discs to the civis 2.93 and drums, the SS rear was not identical to the 9C1 because it is a little wider. The shorter tires on the SS meant though the effective gearing was a hair better. The disc rear brakes were setup wrong from GM they used a drum proportioning valve so they basically only really worked in hard stops, the fix for this cost $20 and is well documented on forum. Once modified to work right they are nice but wont stop faster than the drums first time out but I am sure are more fade resistant because disc cool better. The 17" rims with wider tires on the SS will help cornering but are HEAVY even compared to steel rims and tires, if you want to add them to another model they are under $600 a SET NEW from GM. The 9C1 had some different parts up front to help brake cooling which they certainly do, BUT it does blow water and dirt on the inside face of the rotor contributing to accelerated wear and the air dams have been found to cause some high speed wander. The single biggest advantage to the 9C1 would be the lifetime silicon hoses, the Impala has resale and appearance and the civilian LT1 Caprice had potentially more comforts than the Impala but withoutm the appearance and with lesser initial cost, it is also likely to be a towpackage car meaning a mechanical fan that most of us ditch in favor of dual electric.
I am sure that is more than you were looking fore but the first line in stumpy's post was a crack aimed largely at my attemps to dispell the newbie beliefs that these cars are signifigantly different as the only thing thyat makes an Impala is the WX3 option code the VIN is a Caprice VIN.