Here's the thing though - it really wasn't as straightforward at $17M vs. $21M. The devil is always in the details. Take a look as the structure of these guys' respective contracts:
Armstead:
https://www.spotrac.com/nfl/san-fran...rmstead-16741/
Buckner:
https://www.spotrac.com/nfl/indianap...buckner-18955/
Bucker's contract is frontloaded, a structure which benefits Buckner (he gets lots more money upfront) but would absolutely not have worked for the 49ers, who were already up against the cap and had the personnel in place to make another Super Bowl run.
Armstead's contract, by comparison, is backloaded - he only counts $6M against the cap in 2020 vs. $23.3M for Buckner. He also required a much smaller initial "real" guarantee ($26.65M vs. Buckner's $39.2M), and they've pushed of much of the cap consequences of his deal to later years.
Buckner's contract required a much greater upfront commitment from the Colts, and is fairly straightforward without much financial engineering for the benefit of the team. He gets all of his "real" guarantees in the first two years (2020-2021). After that it looks like he's on a simple year-to-year salary/roster bonus, and the team could theoretically move on from him with few consequences from 2022 and beyond. This gives the Colts a lot of flexibility to restructure if necessary for cap reasons at that time, as I expect we'll be dealing with cap issues in the not-too-distant-future.
So, bottom line, Armstead's deal is much cheaper in real dollars and, perhaps most importantly for the 49ers, gives them $17M in cap relief in 2020 relative to Buckner's deal - much more than the $4M difference referenced in most media reports. He is also cheaper in 2021 cap-wise ($12.5M vs. Buckner's $17M). However, Buckner's deal - while more expensive - is a lot easier to get out of after 2021, and gives the Colts more flexibility in later years. The 49ers deal with Armstead is a good example of buying something on a credit card, as the deal will impact the 49ers cap after 2021 no matter what, even if he isn't on the team anymore. Buckner's won't.
(Side note - this is also an example of how not burning through available cap space can provide you with the ability to pull off a deal that perhaps very few other teams could have.)