Which one should you use for your next project?
Yes, smartphones have greatly improved image and video quality, so much so that the iPhone has even been used on movie productions and commercials, see here. But will the iPhone make professional cameras obsolete? In short, no..but in the future, maybe yes?
Cell phones use lossy video codecs (MPEG, AVC, H.264). This is great if you are uploading videos straight to your social media pages, but for getting professional looking material? Most likely not. (Although Apple is said to be in the works of bringing ProRes to the iPhone, see here).
The big difference with using prosumer cameras, or even professional ones for that matter, is that you have more to work with for a better finished result. Higher dynamic range, interchangeable lenses, and RAW footage.
Apple, Samsung, and other smartphone makers now offer multiple lenses within their phones themselves. This is great if you want to get a medium-wide landscape shot or zoom-in close, but nonetheless this does not compare to being able to use different lenses with faster speeds, and greater depth-of-field. Not to mention pro-level audio, high capacity storage, and using high frame rates for slow-motion cuts.
In short, yes cell phones have come a long way in recent years, but prosumer and cinema cameras are here to stay...at least for now.