Depends how one looks at it: INDIVIDUALLY, 3-4 NTs are worth far more than 4-3 DTs because they're far more critical to their respective base D—but there's only one of them on the field, not two, so they're COLLECTIVELY worth less. Paying one person 50% more to do the work of TWO is still much less expensive.
Just going by
Spotrac, it looks like the devil's in the details (as usual.) They have Dareus as highest earning DT, and his $14½M cap hit is right in the middle between 6.75M and $20M. No one's paying All Pro QB nor SB MVP pass rusher money for DTs; Geno Atkins' 2016 cap hit ($10M) is HALF as much.
It's a tough call because ALL choices may be "wrong." The spot's undervalued, IMHO, and the critical space-eating, pocket-collapsing, double-team-commanding anchor of any 3-4, which is why I like having two solid ones to handle fatigue and injury without missing a beat. I still wouldn't pay any NT as much as Von Miller though, because they can't contribute as much, and there's only so much cap space for 53 guys. Inability to replace Sly for <$7M wouldn't make him worth that.
The best "option" is probably obviating the 5th year option by re-signing Sly now and pro-rating the pain—if only because exercising the 5th year option's not a long term solution: It just buys a year to FIND that permanent fix (while the price inflates.) $6.75M for a single season is a lot when we must release or re-sign more than TWO DOZEN guys by this time next year. Remember when people said that didn't include Sly "because we have a 5th year option; he's not going anywhere"?
For perspective, a good but not great starting 3-4 NT went #11 overall the year before we got Sly, so is playing out his club option now: Dontari Poe.