For me, Parrotlet hands down!
But with that being said, p'lets aren't for everyone. I've had cockatiels and they were a lot less "testing you all the time" compared to a p'let. As others mentioned, I would try to visit breeders of both species and see which species you click with.
Cockatiels I find in general, are easier to care for. They are larger, less likely to become accidentally hurt vs a p'let. Cockatiels live longer (with taking into consideration the common things, health, care, environment etc) I found my cockatiels were more easier to train. Mine were less likely to bite, and a lot more touchy feeling, compared to any of my p'lets. They are more common and easier to find also.
However, my p'lets have just been more of a joy. I like their fiesty-ness. I think their colors are so much more vibrant. I like their stocky build and short tail. (I prefer that type of look, over long tails/slimmer bodies)...I find my p'lets are easier to get toys for as some can be made and used from everyday things. But are to small for larger species to use. My p'lets really enjoy their toys and gadgets, where my cockatiels weren't as playful. My p'lets have been way, way more comical. They aren't as wasteful, I've found compared to my cockatiels. My cockatiels were a lot more messy, I think. P'lets are harder to find and more unique of a species. I always have people comment on my birds how they're so small and unique looking. I really could go on for pages regarding p'lets. I think they are great birds. They are to me the best species out there. I really feel no other species compares.
Good luck on your decision! If you pick the bird that best suits you, you wont be unhappy.
Just want to add: My cockatiels were more into being touched, but my p'lets all have been more into being with me. My cockatiels wouldn't overly care if I left the room, they liked almost anyone. My p'lets right away are always wanting to know where I am if they can't see me. When I get home they happily and excitedly always greet me. My cockatiels did to an extent, but nothing like my p'lets. I like that. My p'lets never liked being touched, where my cockatiels could sit for hours, it seemed getting head rubs. But my p'lets love being on my shoulder. My cockatiels did to an extent but nothing like my p'lets. They have sat for hours on my shoulder, contently preening away and just hanging out. I am my p'lets favorite toy. My p'lets are more of a "one person bird", where my cockatiels usually would let anyone handle them. I prefer the "one person bird" style better. Because I have family members that just aren't into birds. My p'lets leave them alone, my cockatiels usually wouldn't.