which connotation gives to a phrase ending with "na" instead of "ne"? is it more informal or what?
"soo da naa" instead of "so desu ne"
「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.
Yousuke: You are going to the library now huh?
Bob: I studied Japanese a lot, right? But, I still don't get it at all.
In your case,
「そうですね」 would roughly mean "Is that so, huh?"