1. Two 500GB's are more reliable than a single 1TB, as if one fails, you have another. If 1TB fails, you would have a heart attack losing so much data.
Only if losing 500GB is acceptable. If any data loss is unacceptable, then the single 1TB drive is more reliable (assuming an equal failure rate). Of course, the best solution in my opinion is 2 1TB drives, one of which could be used for backups.
2. 1TB would officially give you a little bit more storage space than 2x500GB. As we all know that 500GB specification doesn't actually mean 500GB on OS.
They would give you exactly the same amount of space. 500GB actually means 500GB which means 500 billion bytes. 1TB actually means 1TB which means 1 trillion bytes. Unfortunately, Windows does not use the standard metric prefixes, and instead uses base 2 prefixes which are somewhat close. As a result, Windows thinks that tera means 2^40 and giga means 2^30. These numbers are slightly larger than 1 trillion and 1 billion respectively, and as a result, Windows will cause your drives to appear a bit undersized, even though they actually contain the advertised number of bytes.