Chains described as 7 or 8 speed have the same internal and external width as each other, or very nearly. They are interchangeable.
Depending on the model, a more expensive chain can be lighter (due to hollow pins or holes in plates), or have stronger peening for more sideways strength, or a more rust-resistant and shinier finish. These things don't really affect durability, and sideways strength is hardly relevant in normal use.
A new chain would last longer than another if the pitch was less to start with. They are all supposed to be the same, but there are slight variations in practice. (I have a suspicion that Campag chains last longer than SRAM for this reason, but no proof at the moment).
So basically, if you don't care about how shiny your chain looks (and no chain looks all that shiny once it's been used!), and don't mind a few extra grams, just get a cheap model from a respectable make, such as SRAM, Shimano or Campagnolo.
