Other question about rip settings. codec MPEG4 or h264

[satan4u]

I know some of these questions have been answered before the ones I found were old. some going back quite a few years and wasn't sure if much has been changed since then

I am using version 9.1.5.9

1. AUDIO

which is the best audio codec to use?
MP4 gives me 3 options AAC, AC3 and MP3
mkv gives me AAC and AC3

AC3 gives me audio bit rate of up to 640
AAC gives me up to 512
MP3 gives me 320
and what would the best channel setup be?

2. VIDEO

is there any real difference between mp4 and mkv? I know they are both containers and some people proffer mkv over mp4

Codec
if I select H264 the bits/pixel gets set to 0.22 (high quality native resolution)
if I select MPEG4 the bits/pixels gets set to 0.3 (high quality native resolution)
I know higher bits/pixels is better but why does it set the bits/pixel higher for one over the other and which would be the better option to go for?

if I change the resolution higher why do the bits/pixel drop down to 0.15 when in h264 mode but stay at 0.3 when in MPEG4?