I'm here again. My short answer is go for the best upgrade you can afford. We went for Shimano XTR V brakes and quite honestly they are in my humble opinion the best V brakes out there - end of story. We had the Rohloff option of fitting a disc brake but decided to forgo that and see how we went on. Needless to say it never got fitted.
The important thing is get the best type of rims to go with the brakes. We have a Mavic ceramic front rim and as mentioned on the other post the Rigida Andra on the rear with the CSS rim coating. This braking combination is just fantastic, in wet or dry, even when fully loaded. I can recall one or two emergency stops when such has been the suddenness of coming to a stop Jo has been out of her seat and halfway up my back, with some complaints of course.
Think of it like the difference in the braking requirements of a car against the requirements of stopping a truck! You need better quality and stronger brakes on a tandem.