Before you try and determine what group is best buy a cheap one and ride it for a while then go to your local LBS and see if you can take a bike with a very good (expensive) group and try it. If you don't notice a difference stick with the cheaper group. If you ride a lot of miles then at some point the cheaper group will start to give you problems. Then think about investing in the more expensive groups and they will last longer.
Today machine technology ensures that most groups are pretty well built and work well together when they are first assembled. It is time and miles that separate the low and high priced spreads.