By and large, you play better (from a technical aspect) when the bass is up high enough that you cannot notice the position change when you go from sitting to standing (so either sitting position or standing, high position in your pictures above).
This is why most instruction books recommend that you play with it up that high.
But once you have learned how to play, you may find that it is uncomfortable to play while standing.
If that is the case, then I would suggest you lower the bass by an inch or two (and then play for a few weeks to test the height) until you find a comfortable height.
Personally, I tend to play bass in the middle-low position, but I play guitar in the high position.
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The big differences are in the way your hands are relative to the bass guitar.
For the right hand, this can be largely personal preference.
The height of the bass and the arm positing dictate the angle that your hand is to the strings.
Some variants on bass playing, like Victor Wooten's slapping, are said to be easier to execute on a higher-slung bass guitar.
For the left hand, the lower the neck, the more your wrist has to bend to keep the fingers on the fretboard.
This can cause excess strain on the wrist which can be the source of injuries.
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While choosing a position for comfortable playing in the style you like should be the key factor, a lot of bassists (and guitarists) choose a position based on looks.
Heavy metal bands typically go for the very low slung look-
it has a certain cachet and is associated with some very big name bands, whereas jazz bassists often use a higher position.
Huge generalisation, I know, but still broadly true.
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High or low, my bass teacher recommended an upward angle (unlike the first photo, above), because it Makes the nut end of the freboard closer to your body, so the left hand doesn't have to reach or leverage as far.
Compensates for the longer length of the middle finger, so that you can pluck with the first and second fingers more equally.
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There are several factors affecting it.
1 - Convenience.
Play in a high position is much easier to play, in particular on the high notes - 1st and 2nd string.
The high position is ideally suited for such jazz.
To play the low need of training - it's more complicated.
It is very difficult to reach the high notes.
2 - Appearance.
This is true, when you play heavy music, play the low post is much steeper.
At the same time it's all relative.
For example, Alex Webster played a very heavy music in a high position.
http://i.stack.imgur.com/aK2yJ.jpg Train, try to play in different positions and choose the most convenient for you.
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Like others have stated, try not to worry too much about appearance but comfort and ease of getting the bass work done properly.
This could mean "low" or "high" depending on what type of music you play, and/or what particular area of the fret board you use the most.
While most people consider playing "low" bad form as it is associated with "metal acts" or "looking cool," it can be more comfortable if you play with a pick, or if your fretting hand stays primarily in 1-5 positions instead of playing with the instrument "high."
However, playing the instrument "higher" up makes slapping and popping much easier.
I actually have a strap that has two settings that can be adjusted immediately between songs if I need it high (slapping, or playing high notes up on the fret board), or low to keep my picking hand movement based on my shoulder instead of wrist.
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I think the best option here is to set your strap so it's tight when you're sat down but the bass still sits on your knee.
This way the position of the bass is the same when you're standing as when you're sat down.
This also means you'll not have to adjust your playing style to play standing as everything should be the same.
Also when it's set at that height it's in a good place ergonomically to avoid RSI and other issues common with playing as your plucking hand won't be bent at an angle and the tendons can move more freely.
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During some research I've found out that some guitarists use different position of bass-guitar during play:   Play while sitting:   Play while standing, high position:   Play while standing, medium position:   Play while standing, low and (sometimes) very low position:    I want to know - is this just because of music style for bands, or can the sound be different because of guitar position?
How to hold a bass guitar while playing
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Choose a position that is comfortable rather than one based on looks when playing a bass guitar. Whilst there are several factors that affect this choice, it's generally accepted that most people choose the position of their bass guitar based on looks.
Whilst there are several factors that affect this choice, it's generally accepted that most people choose the position of their bass guitar based on looks.