the difference between so desu ne and ii desu ne

so desu ne
This is a way to make someone agree with you.
For example:
ii otenki desu ne? - the weather is good isn't it?
To answer this, simple say "so desu ne". In this context, so desu ne is used to agree with the person. It can be translated to "it is".

ii desu ne
This literally means, it's good isn't it?
This can be used in many ways. Usually when saying that something is good like a certain time to go out and have drinks or dinner, ii desu is used without the ne.
For example:
konban yoku arimasen, demo ashita no ban ii desu
not tonight but tomorrow is good (for me).

Also, the ne can be replaced with ka to ask a direct question.
For example:
Jaa, hachiji ni. ii desu ka?
Then, at 8 o'clock. Is it good [for you]?