There are outstanding rentals in both towns. As with any life choice, what makes for a great selection is the attention to the details. It's critical to work with an agent that will understand your needs extensively so they can advise you well. You don't want an agent that will make decisions for you, but you will want one that lays out the implications of both towns (the vibe, the lifestyle, etc), individual neighborhoods, commute times, school choices, etc so you can make the very best decision for your family.
It's very easy to make pat answers in a formulaic manner and you shouldn't settle for that. I suggest you work with local experts (I've lived in Millburn for 15 years) so you can learn for yourself what a good value is or is not.
Unpopular Realtor Comment Coming: Selecting an agent should be treated as hiring someone for a specific job. Interview prospective agents, learn their qualifications, style, professionalism, education, experience, etc.
Many people settle on the first agent they come across because they feel all agents are alike. When one interviews agents, one quickly learns that not all agents are the same!
Hope this helps