There are advantages and disadvantages to each situation. A junior agent is going to have more time and possibly be able to give you more attention. Whereas a more experienced agent has more experience and might have more established relationships.
Here’s the thing. If you’ve written a good book either agent is going to be able to sell it. It’s really about which agent are you more comfortable working with and which agent is going to be right for you long term. The hard stuff starts after the book has sold.