A bit encoded as H.264 is not the same as a bit encoded as FLV, quality-wise. It’s about 3 times better at the same rate. Plus, the H.264 videos have stereo audio, the fmt=6 versions don’t.

From http://forum.videohelp.com/topic346256-210.html#1826784
On H.264:
Backers claim improved compression means a dramatic improvement in compression over the older H.263 standard at no loss of video quality – a viewer of an H.264 video at 384K bit/sec should see the same sort of picture as somebody viewing a video at 768K bit/sec sent via H.263.

In the above example, the H.263 video is 571 kbit/s and the H.264 video is 440 kbit/s. In theory this means the H.264 version should even be the equivalent of ~880 Kbit/s videoquality of H.263..

fmt 18 / MP4 fmt 6 / HQ FLV
Screen Res. 480 x 270 480 x 360
Bitrate ≅ 512 kbps ≅ 900 kbps
Audio 44.1KHz 128 kbps Stereo 44.1KHz 96 kbps Mono CBR
Frame Rate 24 30
Video Codec h.264 Flash Sorenson
Audio Codec AAC Mp3