Gimbal head selection.. which one

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I need to make an important choice this week about which gimbal head to use primarily for my sigma 120-300mm which is 2.9KG
it has an internal zoom system so hopefully center of gravity wont change much in use

all of these come in a sidemount version too

my price range is up to 200-250 ish

these are what i am considering

Lensmaster @ 150£


Benro GH2 @ 279


Triopo DG1 @170




Side mount version

Benro GH1

 

[413x]

what is it about the side mount that is bad? and what are the benefits?

i saw the manfrotto one but wasnt sure about its design blocking off both sides of the camera