I haven't split my time, but I owned at Kauai Beach Villas for years. Staying right there is so central to everything, you just make a right or left turn outside of the resort, and head to wherever you want to go. If you stay way south or north, you really have to commit to driving somewhere else. Given your choices, I'd say stay at Kauai Beach Villas or Pono Kai for the whole time. Then drive north or south and spend the day. It isn't THAT far away.
Another point about staying central on the island is that stores in Kapa'a, Lihue, and the airport, are very close by. If you stay far north or south, again, it's a commitment to get to "town" for something.
Dave