Fair point. Nina definitely has the quintessential ‘soul’ voice that Marcia never had. That said, the use of strings (inspired) in the Bob and Marcia version gives it something of a Tamla Motown feel, despite the jaunty reggae rhythm (Marvin Gaye, Smoky Robinson, Tammi Terrell and others used strings brilliantly, many times). It's a hybrid, really. But the mansion of soul always did have many rooms.
And this may be heresy, but the Bob and Marcia version is my preferred one. Yes, even over Nina's, the originator's! Just a personal thing. It was the times, it was the age I was, it was the hopefulness of that year, and the way their version just chimed with all of that.