ending na instead of ne, when to use it?

Question

which connotation gives to a phrase ending with "na" instead of "ne"? is it more informal or what?

Particulary saying,

"soo da naa" instead of "so desu ne"

Answer 1

「な」and　「ね」 seem to be more rough sound and generally applies to the male gender but is not necessarily restricted to only males. You can think of it as "huh". e.g.

洋介：今、図書館に行くんだよな。

Yousuke: You are going to the library now huh?

or

ボブ：日本語は、たくさん勉強したけどな。まだ全然わからない。

Bob: I studied Japanese a lot, right? But, I still don't get it at all.

In your case, 「そうだな」 or 「そうですね」 would roughly mean "Is that so, huh?"