If you have a stylus you like drawing with on your iPad, you can use it on your iPhone 6s or 6s Plus.
During my iPhone 6s testing, I came across a wonderful fact for those who like sketching with something other than their finger: Any iPhone or iPad stylus can take full advantage of 3D Touch—which means, yes, you can use Notes's pressure sensitivity (and any other drawing app's) with your favorite pen. Technologically, it makes perfect sense: The iPhone screen is sensing pressure input upon it, so it really shouldn't matter if you're pressing down with your finger or some other sort of capacitative nib.
That said, this isn't going to bring you detail work and drawing as nice as you might get with the impending iPad Pro and Pencil. The iPhone 6s and 6s Plus have much lower screen refresh rates, which means higher latency, and your third-party stylus won't have the pinpoint accuracy on the screen that the Pencil enjoys.
I did a quick 15 minute sketch (timelapsed above for your entertainment) with the Jot Dash on my iPhone 6s, and while the Dash definitely suffers from latency drag and accuracy at times, it was much more pleasurable to sketch with than my finger. Adding pressure to lines with a stylus nib felt as natural as it does with a finger: You don't have to worry about pressing too lightly or too hard and lines not registering. And Notes offers pinch-to-zoom while drawing, so the latency and accuracy issues aren't nearly as bad as they might otherwise be, were you attempting to draw 1:1 on the iPhone's smaller screen.
Regardless of the limitations, I'm delighted to see that styluses can take advantage of the iPhone's pressurized screen—especially on an iPhone 6s Plus. Drawing on the iPhone 6s with the Jot Dash wasn't too cramped with pinch-to-zoom, but drawing on the 6s Plus feels downright luxurious. (And to top that, I love being able to do nice sketches on the go even if I don't have my iPad with me.) It's a pretty neat little bonus for those of us who are drawing-inclined.