RTMP via HTTP on live streaming and vod

What are the major difference(s) between RTMP and HTTP by using AMS?

Under live streaming, if use RTMP via videoplayer.html, the delay is only 1 to 2 second when go via AMS.

But when using HTTP, the live streaming to a desktop via videoplayer.html is at least 10 seconds and to a iPad is at least 25 seconds.   Seemed better by using RTMP for short delay.

If just for internal live broadcast within office by using AMS without any restrictive firewall, should it be use RTMP instead of HTTP?

On other hand, could RTMP be used for IOS devices?