Trade-Offs
There are trade-offs to consider. The rear traveler is more accessible from the helm position and is more efficient in terms of leverage/speed/friction. Mid-boom sheeting on the cabin top clears the cockpit of all the hardware and gets the mainsheet away from your head but is less accessible for single handing and requires more purchase which equals more line in the sheet, plus more blocks equaling more friction, all of which leads to slower trimming. I had stern sheeting on my tiller C-27 and now have mid-boom sheeting on my wheel C-30. As far as sailing goes, I preferred the stern sheeting while the mid-boom sheeting is nicer at anchor. If you race, or like to single hand, stern sheeting is definitely the better choice. Also, the C-27 reacts more quickly to gusts than the C-30 and larger boats so it's nice to be able to dump the main more quickly.