Looking through my list of favorites, I have about equal representation of each type I'd say.
Gag comcis are easier to get into, you can just sit down and read a few and basically figure out pretty quickly that you like it and want to read more in the future. Plotted comics require a little more of an investment because the nature of the writing typically means you're going to take some time to learn about the characters and the story.
Regarding preference of making them, both have their ups and downs. Gag comics are great because you can just churn out a shit ton of ideas in one sitting and then pick whichever one you want from the list to draw out. When I did a gag comic I could draw five, ten, fifteen of them in one day if I was really on a roll. It didn't matter (or seem to matter, rather) if one comic came out shitty or sub-par because whatever, the next one will be better. I also felt like I had more artistic freedom with gag comics because it just felt like consistency wasn't really a key part of it.
While gag comics are fun for me to make, plot comics are more emotionally rewarding, because I've spent so much time with the characters getting to know them that I really kind of give a shit about what happens to them, and it's exciting to approach *That Plot Point* that i've been waiting for when Character FINALLY gets to Verb. Also, it's harder for me to just give up on a plot comic. My gag comics usually "ended" unfinished, literally, like I was midway through drawing the person or even mid dialogue sentence and just got up and walked away and didn't think of it for months, years, "I'm cleaning out my desk, what is this thing-OH MY GOD FIVE YEARS HAVE ELAPSED." A plot comic is on my mind every day, I'm often daydreaming about where I want to take it even on days when I can't physically work on it. It's harder for me to just forget about it and have it fade into the ether.
I'd figure it's something like a bag of chips versus a delicious holiday meal. The gag a day comics, you can eat one chip and know what the rest of the bag is going to taste like (save for like the occasional burnt chip), and you might find yourself sitting down with the bag expecting to only eat a handful, but next thing you know, the bag is almost empty and your hand is sweeping around the oily innards to try and find that last big crumb you're sure is in there. With a long-running plot comic, you feel like you need to sit and really savor each bite and pay attention so that you don't miss out on something. If you just rush through it trying to get to the end, it's not enjoyable.
Goddamn I make this fucking food analogy at least once every two weeks or something
Do not feet infants to honey under one year of age.