What are good back up devices for iMac and MacBook Air

The read / write speed advantage of SSDs is utterly unimportant with Time Machine.

Read Types of disks you can use with Time Machine on Mac - Apple Support. Whatever you use, get enough of them for a sufficient number of redundant backup devices. Since any device can fail at any time, that means a minimum of two backup devices.

Since two devices will are certain to be in the the same physical proximity least some time, they can both become simultaneously lost through a catastrophic event or theft. That means a minimum of three devices, with one of them geographically separate from the other two at all times.

Three redundant, inexpensive, disposable, "slow" backup drives. When one fails (and they all fail... eventually) throw it out. Unlike SSDs, most hard disk drives give at least some warning before they fail completely, but when they do — guess what — you're back down to only two.

Four backup drives get very close to a disaster-proof backup strategy.

That will really depend on if this is the only backup destination you are planning to use with these Macs. HDDs are still the preferred media (I have to slightly disagree with Rudegar on this) over SSDs for the primary purpose of using for backups.

Regardless, you will want to have as a minimum 2.5 - 3x the total combined storage capacity of both of your Macs for this backup destination ... even if Time Machine does perform backup cleanup.

Now, the other important issue is: How critical are the data on these drives to you? If they are critical backing up to a single external drive will not leave you any room for error should that external drive fail.

The other consideration is your budget. If you can afford it, a dedicated NAS would be the better solution having a single external drive. Also that single drive would have to be connected/reconnected between devices each hour for Time Machine to use them from each Mac. Not a practical way to go.

What's your budget and what app are you using for backups? Does either device have Thunderbolt, USB-c? Are they "talking" to each other? What strategy are you currently using for your backups?

A lot of questions however all plays into what devices work best for your application.

Agree with Rudegar on Thunderbolt or usb4

Yes....backups are critical. That is why some users and businesses keep more than one set of backups. No one ever regretted having an extra backup. Some users even keep a backup offsite in case of theft, fire, flood, etc.

would likely be a usb4 or thunderbolt external harddisk to use with timemachine, if you can afford it then SSD disks are faster