From Nelson both routes take about the same amount of time, and Lewis Pass is a little shorter in distance than via Blenheim & Kaikoura. From Motueka, the inland route (over Lewis Pass) is quicker and shorter.

Both routes are nice - inland you have plains and valleys, a couple of mountain passes, quite empty countryside for part and more populated at the start and end. Coastal you have a couple of small hills just out of Nelson then valleys, road is right on the coast for a long stretch with mountains the other side of the train tracks, then rolling countryside before the plains.

If driving in middle of winter the coastal road may be better (less risk of closure due to snow).

Both roads have nice places to stop (for a walk, a meal or overnight).