Right now, definately the pick.
No matter what we do this offseason, we have precious little cap space for improving our team via free agency.
We have the assets to make some small improvements via trades, but not enough to make us a contender - adding a guy like Kevin Love won't achieve that for us, yet will cost us half our assets to acquire.
Through the draft we can essentially get a talented young player for nothing. We don't need to give up any of out own guys, and we don't have to waste massive amounts of cap space...and Danny gets complete freedom to choose who he wants out of the 25 or so guys who remain on the draft board at that time.
Plus in any lottery there is always a slight chance that the ping-pong balls could fall our way, and we may end up with a top 3 pick. If that happens we can draft somebody like Embiid, Exum, Parker or Wiggins. Some of those guys have more potential than others, but ever one of them could improve this team right now.
If we added one of those exceptional draft picks AND we managed to also pull of a trade to being an All-Star calibre player over...that then leaves us with a lineup that starts with Rondo, a potential future all star, an actual current all-star.
Plus on top of that we'd still have some excellent talent to fill in the roster, such as Sully, Bradley and Olynyk (who all have high upsdide). We'd also have tons of picks in the future, which would allow us to bring in young talent every year for as far forward as I can see. With Brooklyn looking like they will be terrible after next year, a number of those could be lottery picks too.
Basically if we sacrifice this season, that puts us on tracks to be very, very, very good two seasons from now when out new talent develops, and our big contracts (like Bass, Wallace, Green) have expired.