Great input, thanks guys. Always nice to have the locals weighing in with differing opinions. We've decided to go with the West Coast route. It looks like we'll be able to fit in a few nice hikes along the way. I've booked our first night right in Arthur's Pass, Google Maps predicts under 2 hours so even doubling that makes it a pretty easy first day's drive as far as I'm concerned. The next two days before we need to arrive in Queenstown are still up in the air. I'm looking at kiwibigdave's suggestions, but leaning towards spending both nights in the same place.

I've also booked us the two day (one night) kayak trip in Doubtful Sound for our time after Queenstown. Looking forward to that, anticipating rain and/or sandflies.

Too many things to see, too little time!