The latest IPhones have the second best small hd screens currently in circulation. They really are very good. The only device I know of with a better screen in that size range in the Samsung Galaxy S5 with the OLED screen. OLED is currently the best display technology available.
You want to be able to see your target well as you record and higher quality screens have higher contrast ratios which shows more fine detail. More contrast and a higher frame rate can do more for clarity than more pixels on screen.
In terms of recording quality, I've never found smartphone cameras to be as good as a high quality dedicated camera. On the other hand, I haven't found them to be much worse than many low cost dedicated cameras that use similar small lens sizes. Small lenses really limit light.
The obvious point is that that there is broad range of dedicated cameras so a trip to Best Buy to see for yourself is a good idea.
