The stories do not have much in common besides being romance and fantasy but the male and female leads are the same for both dramas, so if you like the pairing you should watch both dramas.

Although the stories are completely unrelated both dramas star Lee Seung Gi and Suzy as leads, if you like the pairing you will want to watch both dramas.

The dramas are nothing alike in feel and themes but the only thing that could take me out of my Ashes of Love slump was Bloody Romance. Watch one after the other and you won't regret.

Both are about a girl that travels to the past and falls in love. In OME, the female lead travels through time and falls in love with an emperor while in TEL the FL wakes up in the body of another girl.

Both are web dramas and are funny and full of eye candy

The stories are very different but the main focus of both dramas is the romance between the leads.

In both the leads knew each other for a long time and reconnect some time later. Both dramas start with them seeing each other for the first time after a long time. In both they have to secretly date because of some opposition. They are both full of cute moments between the leads and both couples can be playful with each other.

Really cute romances overall.

Both stories are about grim reapers and souls. Black is a drama about a grim reaper that goes to the human realm in search of a soul he lost in the process he meddles with human affairs and remembers his past as a human. In "Along With the Gods" we see more of the other part of the grim reapers' job, guiding the souls through their next journey and eventual reincarnation, the main grim reaper, Gang Rim also interferes with the human "realm" because he has memories of his past self... In both there is a "mystery" that gets uncovered, although in AWtG is not as complex as in Black.
The two stories are different but have the same gist and give similar feelings. They both have great CGI and awesome acting.

Sound of the Desert is similar to Eternal Love in that:

-Female lead is in love with someone else first.
-Male Lead is completely in love with her and is super open about it. Affectionate, wicked, naughty.
-Second female lead is super hateful, although in SotD the second female is not in love with the male lead.
-Passionate kisses, although in both the romance takes a while to start.
-There's no magic or fantasy in SotD
-The male leads always wear black.
-Overall the romance gives the same vibe.
-There are a few more similarities but I feel they would be spoilers.