Generally speaking price is the biggest difference.  I would say though most companys ball have slightly weaker cover or older technology covers but this is not always the case.  An example I can give is Brunswick's use activator(newer technology) on their infeno's and PK18(been around for years) on their mid price balls, but then again they stuck activator on the zone classic which is upper mid priced.  Another difference is sometimes in the complexity of the core,  I use track for this example as you can see the morpheus and all its variations in there upper end stuff and they still use light bulbs and such variations in their mid priced stuff.  Some bowlers with a high powered game may not even need high performance equipment, they may get all they need from the less exotic cover/core combo's,  there is one guy around my local house that just can't use upper end stuff because of his style.
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I should just quit bowling, oh wait I already tried that.
Now that I am back and my Saws are sharp again, I am ready to cut some wood.