Currently in Iceland and did both day trips. (Very easy to drive here btw.) RS says these trips are a tie for #1 but our whole family liked the South Coast scenery better. Weather seems consistently better in south coast too. Just an fyi on our opinion!
I too prefer the south coast over the GC. We have been 4 times so far, and still visit both on all of our trips. The weather though, it just depends. The south coast weather can be pretty nasty. Much depends on time of year of course, but as you have probably found out the weather there can change quite drastically every 5 minutes.
Yes, I’ve heard many climates described as, “if you don’t like the weather, wait 5 minutes” but in Iceland it’s actually true!
I bought a mug there on our first trip back in 2015 and it shows a clock and every five minutes it has a different weather event;)