Take a look at www.morsex.com
to see a good selection of paddles of all types.
With a single paddle, move paddle in one direction to get a stream of dots, move it in the other direction to get a stream of dashes. Tap the paddle in the appropriate direction for a single dot or dash. So to send an 'F' ( ..-. ), move paddle to send 2 dots, then the other direction for a dash, back again for final dot; a total of 3 movements. Similarly, a 'C' ( -.-. ) would take 4 movements.
A dual paddle has 2 paddles ! One for a stream of dots and one for a stream of dashes. However if the paddles are 'squeezed' together you will get a stream of alternating dots and dashes. When holding the dot paddle to get a stream of dots, tapping the dash paddle will insert a dash into the dot stream (and vice-versa).
So now, to send an 'F', move the dot paddle for 2 dots, keep it held in, tap the dash paddle and release. A total of 2 movements. A 'C' would be sent by squeezing both paddles, with the dash paddle very slightly before the dot paddle; effectively a single movement.
Which to choose ? Entirely personal preferance. A dual paddle will eventually let you send faster code and will be less tiring. However a single paddle is capable of being used way beyond the speeds that the majority of hams use on the air. A dual paddle is a little bit more tricky to get used to and need a slightly more delicate touch, but not so much that it should sway you one way or the other.
See if you can try both sorts at a local club or ham store. Whatever you get, go for quality. A good paddle will last a lifetime and is a joy to use. A cheap paddle is a pain in the bottom and will put you off CW for life.
Sorry for going on a bit !
73 Steve, G4GXL