HA with hardware or software?

Hi,
I'm running an ASG220rev2. It's getting old and we are outgrowing it. I'm looking at two options... Going with a new ASG220 and using the old ASG220 in HA as the slave. Or changing over to a software license.

Does HA work between different ASG220 revisions?

How does software HA work? Is it an additional cost? Do I need to run identical hardware?

I don't know the costs but I'm leaning towards a software license because then I can have redundant HDDs and PSUs.

BTW, I have 50 users on my ASG220. It has Network Security and Web Security and I use IPS, AV scanning, Web filtering, and App control. Plus I have a couple SSL VPN users and a satellite office on a RED10 with 5 users.

Thanks,
Ben