In terms of pure scrummaging ability I think Kepu is ahead of Slipper but not by much. Kepu still has a habit of giving away penalties, particularly when he first comes onto the field.
They are close enough that some days Slipper will scrummage better than Kepu.
I think Slipper is consistently better around the park though.
In terms of balance, I'm not against starting Robinson and Slipper and finishing with Sio and Kepu. Start with your best scrummaging LHP and best around the field THP and finish with your best around the field LHP and best scrummaging THP.
That to me seems like a good compromise.
They are close enough that some days Slipper will scrummage better than Kepu.
I think Slipper is consistently better around the park though.
In terms of balance, I'm not against starting Robinson and Slipper and finishing with Sio and Kepu. Start with your best scrummaging LHP and best around the field THP and finish with your best around the field LHP and best scrummaging THP.
That to me seems like a good compromise.