Platinum gives you a free companion flight after the first year. Other than that, they're pretty similar. You can have more than 1 of the same card and they may send an offer to upgrade. You can also request one after the first year, but it probably won't be as good as one they send you.
Also, here's an assist in trying to reach @K-in-Boston
If you are like me, you don't even have to use it for anything else. Feel free to still use your other cards for rewards, the way you see fit. Anyway, the reason for going Platinum is the Companion Certificate. This benefit alone more than pays for the card (I use mine on around $600 tickets). It never made sense to me to get the gold, unless you will not use this benefit.
I got Delta Gold for sub, i will downgrade it to blue after a year. I am getting gold status from flying, and I travel very light. So this card's perks are not important for me.
When I travel with companion, its almost always overseas. The companion cert of platinum is, in most cases, useless to me.
Just a word of caution - the email offers from Delta aren't based on a pre-qualification. Check Card offers on Amex site to see if you're likely to actually get approved.
(I get them all the time and know from experience I won't be approved)
Sorry I'm so late to chime in. I feel loved.
Responses so far are great. Even living in a hub (and now that BOS has officially been named one, I guess I do too), having Delta cards can still benefit you even if there might be 8 Diamond Medallions on many of your flights!
Certainly, if you can use the companion certificate each year, Platinum or Reserve make perfect sense since usually that makes your net AF a negative number just after the certificate. On your international tickets, do the math and see if it makes more sense toward status and mileage earnings to book partner or codeshares instead of Delta-marketed flights, but credit your SkyMiles account with the ticket you purchased from Air France, KLM, Virgin, or whoever. Gold Medallion or higher can really pay off with the perks on international itineraries as you'll have access to SkyTeam lounges, expedited security, expedited check-in, etc. even when flying in economy.
As long as you're not opposed to new accounts, I would always highly suggest applying for new Delta cards rather than upgrading since the elevated SUBs are much better than even the best upgrade offers. As a rule, those are once in a lifetime so make sure you can get the highest SUB possible. DW can apply for as many cards as she wants; even being an AU on your account(s) will not make her ineligible for SUBs.
For most people I would agree with the sentiment that the Delta cards are generally best used for the perks and not for spend. But of course if you do, there's the MQD waiver up to PM status and Mileage Boost for Platinum and Reserve cards. I've personally decided to hit $30k and stop on Delta Reserve this year. Hitting PM for 2020 (would have been short this year on MQDs) and knowing that I'll end up using the mileage earned on a high value redemption made that an easy decision for me.