I'm thinking about getting extension tubes for my Nikon D3100 but can't figure out what the difference is between the following two (apart from price):

Kenko DG Auto Extension Tube Set for the Nikon AF Mount
Price: £123.25
12 mm, 20 mm, and 36 mm

Polaroid Auto Focus DG Macro Extension Tube Set
Price: £59.99
12 mm, 20 mm, and 36 mm

Both have got contacts so I will have some sort of control over the focus.

The lenses that I have got are the 18-55mm and the 35mm f1.8 prime.

Thanks,