Some good responses on here. There's alot of saltwater experience it looks like, and lots of folks that bailed as soon as alot of the trendy aquaculturists became a thing, and leds took over the crown of the lighting world (from a sales standpoint anyway).
I'm like many, started off with a fish tank, then graduated to a planted tank, then graduated to a FOWLR saltwater tank, then to a reef tank. it's a natural progression.
Generally speaking, reef tanks will ultimately be alot more expensive. Organism for organism the salty ones just flat out cost more. Even the nicest and rarest of plants, generally only cost about as much as middle of the road corals. And while you may find some monster tank sized fish and stingrays, or certain discus, that cost into the hundreds, it's common place for alot of reef fish to cost alot more than that, even some nano tank sized fish.
It CAN be cheap to start up a saltwater tank, but on a long enough timeline, it generally becomes more expensive.
I think cost aside the biggest challenge for a long time planted tank keeper, who decides to try their hand at coral keeping, is the patience of it. I've known many many many expert level planted tank keepers that struggle mightily with saltwater tanks and coral keeping. Not because they lack the skills or intelligence, but they lack the patience. Even the easiest of soft corals will appreciate a mature tank.
Algae is another big deterrent for crossover hobbyists. In a planted tank, algae means you have a problem and it can be corrected easily, often overnight, without any detriment to anything else in the tank. In saltwater tank, algae doesnt mean you have a problem, but that is hard to grasp. Algae is a massive portion of the overall biomass and food chain on a natural reef. It is REALLY REALLY good at surviving and thriving in ALL conditions. A young saltwater tank will have several tank wide hideous looking algae blooms 99% of the time, over the first year. If you happen to avoid them...you either used very high quality and mature live rock (like the above mentioned TBS rock), or you are running your tank in the dark. Algae happens, and you have to just accept it and keep up with solid husbandry and it will be gone all on its own with moderate intervention.
The maturation process of a reef tank is infintely more complex as well. You don't have to really do anything but wait, no gear to buy or science to perform for your tank to mature, just time and patience. You're not just trying to let your filter media colonize with nitrifying bacteria. You're trying to get your rocks and sand to colonize with nitrifying bacteria, develop beneficial anaerobic zones for DEnitrifying bacteria, build up naturally breeding populations of phytoplankton, zooplankton, copepods, amphipods, and hundreds of species of sessile invertebrates, dozens and dozens of species of macro and micro algae and biofilm to colonize the rocks and sand, to ultimately form a food chain that goes from microscopic, up to fish sized. That takes months and months longer than just having a mature filter media. That process is what alot of new hobbyists get hung up on and ultimately really discouraged about.
If I have no ammonia, and my tank is now 6 weeks old, why are my corals dying and why do I have ugly green hair algae and cyanobacteria everywhere....Well...because that's what happens, that's the process.
Once you've done it a few times and have your gear and maintenance regiment in place, you can manage a new tank with corals because you'll know what to look for and what to provide for them while a tank is still in its infancy, but I can tell you, a mature tank that has been running a couple years is WAY more forgiving than a brand new one. Any of you aquarists looking to jump into the reef side please keep that in mind. In a planted tank, you can set it up and have it looking spectacular...contest winning, in a matter of days. As long as you start with enough plant mass. It just doesnt work like that in a reef tank.
Those are just my thoughts. I'm not an expert, but these musings have been my own personal experience. I currently run a 75 gallon dutch inspired planted tank, that has had it's ups and downs and generally looks "ok" right now. It's 7 months old roughly, but i've had this same tank and done probably a dozen tear down and rebuilds since 2007. I also have a 90 gallon reef with 40 gallon sump that I run which is about 1.5 years old. Previously I had a smaller tank that ran for a few years. I havent been in the hobby for decades like some on here, but long enough to "get" the differences between the two sides of the hobby.