Love Tanev, but I would prefer to keep Toffoli when taking in consderation of both injuries and age curve.
This is the crux of the problem. It's age/injuries vs. positional need.
Tanev addresses a huge positional weakness for the team - both for the present and upcoming years (we don't know if Rathbone will re-sign yet, which leaves Juolevi, Woo, Rafferty, and maybe Tryamkin as prospects in the pipeline....none of these seem to be top pairing, shut-down dmen). If we're talking immediate help, Tanev can't be replaced internally. But his injury history is certainly a concern, and if he were to sign a ~5 year contract, there's a good chance he doesn't play a lot of the games and his play declines in the last couple of years.
Toffoli's younger age fits with this core better, but as another poster mentioned, having Pod and Hoglander in the pipeline makes him more expendable in the near future.
I guess it makes the most sense to re-sign Toffoli and then trade him for a top 4 dman once Pod/Hoglander are ready to take his spot. But I think we'll miss the playoffs without an adequate Tanev replacement for next season, so this option isn't great for the short-term. And I don't have faith in Benning acquiring a top 4 dman via trade when his history includes Gudbranson, Sbisa, Larsen, Clendening, Pouliot, and Pedan.