The difference between 6 and 7 string guitars is 7 string is used in a lot of jazz and solo guitar if it is your first guitar you should definitly get a 6 string
The main difference is the extra string on the 7 string guitar. This is usually tuned to a low B, or C, C# or even a low A. For metal this will give you the benefit of playing low and heavy riffs, while keeping the other 6 strings the same if you already play a normal 6 string guitar.
It is a little bit more difficult to play because of the wider neck of a 7 string guitar. You also need to lay down some more cash because most 7 strings are not the cheap guitars which most beginners start out with.