He had a 114 OPS+ with regular playing time last year. Since being back in AAA with regular plate appearances, he's got a .366 wOBA. He also doesn't hit arbitration till 2022. He's not a prospect to build a core around, but if you're entering a rebuild like the Giants you could do way worse than having him in the OF rotation.
I can't imagine the market for Smith being all that robust considering the service time. I figure the Cubs will be interested, but they're pretty strapped right now on their farm. They'd likely also be offering damaged goods like Ian Happ, I presume.
Cave also had a .363 BABIP last year.
Smith is a pending FA but he is well regarded and pitching great right now. A LHP who dominates both sides. He might be the Giants best trade chip, so I think they need to aim for upside on his return.
Don't get me wrong, Cave is a useful player and the Giants would benefit by having him. But they can't spend Smith to get a guy like that, or their rebuild will go nowhere -- they need to find their Cave types just like how the Twins did (DFA and minor trade situations).