I think it depends on what you’re using them for. I assume you’re not going IFR (or else you would probably be keeping at least one Garmin always current).
For one-time upgrades, I’m not aware of anything other than just calling jepp and arnav and getting a one-time upgrade card. (Note that Arnav upgrade to V920AG software is free and includes an updated database as well as the improved software).
I don’t think Jepp offers one-time Skybound datawriter purchases (but you could check, if you wanted to do this).
So I think you really just have one choice – calling and getting an upgrade.
Now - if you want to start keeping the garmins constantly current (i.e. legal for IFR), then you have some choices like whether you want to exchange cards or just download the new database over the internet. The former choice is more expensive but probably more hassle-free. the latter requires a Win98 machine with a PC Card [PCMCIA, i.e. laptop card] port and internet access, but is cheaper.
Then you have to decide whether you want full USA or only a portion of USA, or worldwide, or north america [or other regional coverages if you’re not in the US!]
Are you flying IFR or VFR, and do you want to start getting the updates every month or just a one-time upgrade; that’s the main questions you need to answer, I think!