If literary agents work like theatrical agents do, there's a trade off.
'Well established' will be less able to/likely to concentrate on you as an individual since the bulk of their income (and therefore attention) will be focused on their already-established, successful, lucrative clients. But they will have lots of ready-to-go contacts and relationships with the people who ultimately will employ your services (producers and casting directors / publishers) and therefore are more likely to have access to a wider spread of the available opportunities.
'Young and hungry' are more likely to concentrate their attention and efforts on you since they'll be more dependent on your success to improve their own income. The downside, though, is that they may not have the same industry 'clout' or ready-made access to the bigger players in terms of placing you. They (an you) may not even see a lot of the opportunities that are out there, since the established agencies will have done the deals over a lunch somewhere before the smaller agencies even know about them (if, indeed, they ever do).
The ideal (at least in the acting business) was always to try and get taken on by someone who used to be with a bigger, more established agency (and thus has the contacts) but has recently left to start up on their own (and therefore has a smaller and developing client list). These 'unicorns' embody the best of both worlds... and are consequently a) as rare as dragon eggs, and b) hugely over-subscribed!