Neither is "better" overall-they're just better for different jobs! Let's break it down so you can pick the right one for what you're doing.
First, plastic spackle knives. These are the gentle giants-great for delicate jobs and small fixes. They're lightweight, so your hand won't cramp up after 10 minutes of spreading spackle. And since plastic is soft, it won't scratch surfaces like painted wood, glass, or even car paint (I've used mine to scrape bug guts off my car-no scratches!). They're perfect for tiny jobs: nail holes, small dents, or smoothing painter's tape.
Plus, they're cheap-you can grab a 3-pack for under $10. But here's the catch: They're not tough. If you press too hard (like when spreading thick joint compound) or use them for scraping old paint, the blade will bend or crack. I broke one last month trying to scrape a sticky label off a wooden shelf-oops.
Now, metal spackle knives. These are the workhorses-built for sturdiness and bigger jobs. They have stiff blades that can handle pressure, so they're ideal for medium to large drywall patches, spreading thick joint compound, or even scraping off small amounts of old paint.
They last way longer than plastic, too-my dad has a metal one that's 10 years old and still works. But they're heavier, so your hand might get tired faster. And since metal is hard, it can scratch delicate surfaces-never use a metal knife on painted furniture or glass unless you want ugly marks. They're also a little more expensive, but worth it if you do a lot of DIY.
So, which to choose? Plastic for small, delicate jobs (and if you're on a budget). Metal for bigger, tougher jobs (and if you want something that lasts). I keep both in my toolbox-you never know when you'll need one or the other!
