Is the 120 FPS video recording's quality 720P?
According to this lengthy review the recording is at 720p.

The specific recording details are:


H.264 codec
1280 x 720 resolution
Millions of colours
AAC, Mono Audio @ 44.100 kHz
120.15 FPS
29.72 mbits/s


This all matches the recording quality of a 'regular' iPhone video @ 30FPS, apart from the data rate which will vary from recording to recording.

Note that the iPhone can record 30 fps @ 1080p, according to the apple tech specs which means that the higher frame rate is at a lower resolution.