I bought my original blackbird with the line/strap suspension, and my next one with the buckles and an extra set of buckles for the first blackbird. After that, I switched both blackbirds out for whoopee slings, and now I'm actually considering going back to the buckles for the blackbirds. Both ridgerunners I bought, I ordered with the buckles as well.
To me, the biggest benefit of the all-in-one straps/buckles system is that your tree hugger and your suspension are one in the same. That means that, if you have huge trees close together, or little itty bitty trees really far apart, either way your suspension is as flexible as possible for both situations.
I've been in situations with my whoopee slings where I wanted to hang from two trees close together, that were also small in diameter, and I had to shorten the tree huggers, either by using a toggle (I hate toggles) or by wrapping the strap around the tree a few times (somewhat squirelly with Dutch clips).
I'm leaning in the direction of thinking the mild weight penalty for the all-strap suspension is worth it for the convenience and lack of hassle.
Also, regarding the line/strap: I hate it. Being able to lay in the hammock, decide you don't quite like where it's at, and easily tweak your suspension is a huge bonus of whoopie slings / straps-n-buckles. With a knot-based suspension, you have to keep re-tying knots. I will never screw around with that silliness again, now that there are so many better options for suspensions.