Both are military survival competitions, although the concept is a little different
Both have very similar drills/games/missions
I think The Soldiers was actually spun off from the Iron Squad because of it's success.
I think Iron Squad is much better imo as you get to know the contestants more, there's MC panelists, and more heart, but if you're in the mood for more military competitions, then try the Soldiers.

Both have Kim Seul Gi & Ahn Hyo Seop
Both are a short mini romcom
Both are lighthearted, silly, cute & funny

Both are created by Studio Dragon so they have the same vibe
Both have a haunting darkness to it
Both FL's are emotionally wounded with cold exteriors
Both MLs are caregiver types
Both FL's have fab wardrobe
Both FL's homes are grandiose

Honestly the stories are so similar, it'd be better off listing how they're not similar.
Both essentially have the same storyline.
FL is crushin hard on the ML
FL follows the ML around
ML is an intelligent nonchalant type with a female fan following
FL is a bubbly warm no so bright girl who tries hard, but keeps getting herself into trouble
and the list could go on forever

Both star Nam Ji Hyun
Both have cute MLs who have amazing chemistry with NJH
Both involve cohabitation
Both MLs get easily jealous

Can't say the stories are similar at all, but if you like NJH and are looking for a similar chemistry w/ a ML, I would check out Suspicious Partner

Both have a cute spunky FL in high school and a gentle eyed ML.
Both MLs learn something from the FL and in turn falls for them
Both have a sort of parallel world aspect
Both FL are funny

While the stories can't really be compared, both have a similar feeling that I think if you liked one, you'd like the other. Plus Splash Splash Love is only 2 episodes so it's an easy watch.

Both written by the Hong Sisters so there's a similar mix of darkness, humor, yearning, banter, and love
Both have leads with ancient supernatural powers to a high degree
Both leads use their supernatural powers to protect their love interest
Both involve a sad love story
Both involve the seeing of ghosts
Both are opposites attract

Both dramas star Hyun Bin as a rich businessman who likes to banter with the female lead.
Both dramas have female leads who have strong personalities and can hold their own in the banter.
Both start with the female lead disliking the male lead, but eventually falling in love.
Both are funny