Marlborough has far more wineries than Nelson (or any other NZ region, in fact) and a pretty good mix of small boutique wineries and much larger producers. Best source of info is here:
http://www.wine-marlborough.co.nz/wine-trail/
If you opt for Marlborough, Blenheim and Renwick are so close together that it makes no real difference; Blenheim is a small and sleepy town (30K people) but is the regional centre and at least has a few reasonable restaurants and some unremarkable bars/pubs, while Renwick is absolutely tiny and has nothing going on. Blenheim has a far bigger range of accommodation, plus there are some nice properties to stay at that are scattered around the countryside and among the vineyards themselves. Nelson is about twice the size of Blenheim, and quite a bit more sophisticated, but is 90 minutes drive away, some of which is very hilly and winding, and would *not* be fun after a day of wine tasting. Purely for the quality and variety of wine, I'd suggest Marlborough. Nelson is a much more local experience, though still an enjoyable one, and does have some standout producers (e.g. Neudorf).
Originally Posted by
number_6
You'd first need to work out which wineries will be open for visiting on the date that you want. Most are closed during the week or by appointment only or you need to be in the trade. It isn't like some countries where open for tours/tasting all the time. Your best bet would be a B&B stay at a winery or affiliated with a winery.
That's mostly true, but there's a decent number in Marlborough that stay open year round, and there's only so many wineries you visit in a day anyway. However many just have a cellar door and tasting facilities - you'd want to call ahead to find out whether you can get a tour.
One place that may be of interest is the Brancott Heritage Centre, a hillside facility with impressive views and run by the country's largest wine producer, which is a kind of education/interpretation centre with various 'experiences' and tutored tasting options, and an onsite restaurant. It's open year round.
http://www.brancottestate.com/our-home/cellar-door