1 Are there any diffrences between the standard culture/trade structures and thier Starbase versions? Is one more cost effective than the other? Is one more powerfull than the other, etc. ?
They are meant to be sort of competitive for their costs. Starbases generate more trade and more culture than a single analogous structure, but those upgrade ranks also cost more. I don't think anyone's done the exact math yet.
2 Is having say, a Tradeport and a Starbase with a Tradeport upgrade both on the same planet redundant, or do the two structures stack their effects together in some beneficial way (same question for culture too) ?
They do stack the same way as just building another building would stack.