I would choose Blenheim over Nelson any day. The climate is better in Blenheim, but the real reason is that Nelson is so Conservative in everything. The wacky religions just love that area. Marlborough is so much more civilized it has a vibrant community (might be the grapes) better Theatre offerings if you are into Live Theatre, and so many choices of where to live from Picton in North to Awatere Valley in South.
You have the beautiful Queen Charlotte Sounds, Port Underwood, The Centre of NZ Wine industry. Whats not to love about that.