Picking between an up cut or down cut CNC router bit can be a daunting task especially you are just starting out. Without going into to much detail here is a little info to help you choose the correct bit.
Up-Cut Bits Pros:
Very efficient at keeping chips from building up in the cut area.
Build up of chips in the cut area can cause bind
ing possibly breaking the bit.
Up-Cut Bits Cons:
Will try to lift your application off the table.
Will leave a rough frayed top edge on wood.
Down-Cut Bits Pros:
Leaves a nice clean edge.
Pushes application down towards table.
Down-Cut Bits Cons:
Not very efficient at removing chips from cut area.
Which can lead to a build up of chips, causing binding possibly breaking the bit.