You're asking this on FT?!?! Aren't most here comfortable with packing light (don't need much for Hawaii anyways) and switching between hotels (maximize stay counts) at the drop of a hat?

And as gregorygrady pointed out and which I agree, you may actually end up saving valuable driving time by splitting the visit. The traffic in Kapa'a and Lihue can be a real drag on one's vacation so why go through it repeatedly in order to visit the other side of the island. The OP will already be short on time by island hopping. BTW, the helicopter suggestion by high_flyer at the onset of Kauai visit is an excellent tip to get a breathtaking overview (literally and figuratively) of the island! Some are doors-off like Inter-Island and Jack Harter.

Besides, GH Kauai offers much more than just its pools and lagoon, which by the way are one of the finest on the islands, second IMO only to Grand Wailea. It has an outstanding spa (Anara), Grand Club Lounge, better weather and terrific on-site dining options (the romantic Tidepools and Dondero's) and bar (Stevenson's Library). One caveat: I've yet to try out Jean-Georges Vongerichten's Kauai Grill or the new on-site spa at the StR Princeville to offer comparisions; the old Princeville's dining options left much to be desired and spa was off-site. I'm just praying they didn't change the Living Room all that much.