I assume that AccountNo is an ID that you will use only as a key or selection field, and that you will never make any arithmetic calculations with it. If so, there are no longer any major advantages with having it as a number. (Except that a number takes less space in RAM than a string.)
Further, you have an account number that cannot be represented exactly as a number. You can use Evaluate() to assign a numeric value to it, but you will lose information. For instance, the following two account numbers will be assigned the same numeric value:
So, the advantage with storing it as text, is that you don't lose any information: The two account numbers above will still be two separate account numbers.