The short answer is: Pick either one. Many players have veered toward Shadow Wardens because the daily quest chain takes a little less time to complete. When you've completed each 150 Mark of the World Tree quests (both Shadow Wardens and Druids of the Talon), you will have a choice each day on which quests you want to do.
The long answer: When you've accumulated your 150 Marks of the World Tree during your first week or so of running dailies in the Molten front, you'll have a decision to make. Much like Rebecca Black's internationally renowned hit "Friday", you may be thinking to yourself -- Which one do I choose? Chairman Zillionaire first suggests that you take a long, hard look at your life. Then, follow this advice: If you would choose to sit in the front seat, then pick Shadow Wardens. If, rather, you would choose to sit in the back seat, pick Druids of the Talon. Now, there may be a connotation of "front being better than back" laced here, but sincerely, you shouldn't think of it that way. If you're in the front seat, you've got a better view. If you're in the back seat, you'll be with your friends, so you don't need the better view.
What we're saying is that this is a win/win situation. Whether you choose to start with Druids of the Talon or the Shadow Wardens, you will need to complete the other set in about a week too. So, you can ride to school in the back seat with your friends, but then you can ride home in the front seat with the awesome view. Accumulating your next 150 Marks of the World Tree will go much faster than accumulating the first 150 because more daily quests will be open to you. And before you know it, it'll be Friday again, which comes right after Thursday and right before Saturday, and you'll be faced with new challenges of the Firelands.
How to get to Firelands Entrance
Firelands Dailies Guide
Review of Firelands Dailies Plus Some Inspiration
Shadow Wardens vs. Druids of the Talon - Which one Do I Choose?
Calling the Ancients, Filling the Moonwell, or Additional Armaments: Which to choose?