Well, picking out an agency can be an involved process. You're going to get all kinds of recommendations as to which one is the best, etc. I've been traveling for about 9 years, dozens of contracts and have used several agencies. I have no preference to any one agency. It comes down to which one can get you a contract and how much they are going to pay. Some contracts are nice, some are awful.
It used to be that a hospital would put out a need and most of the agencies all had the same hospital need and it would just be deciding which one had the best rate of pay for hourly and per diems. Now, the mid level vender management companies have taken over and they take their fee off of the top before your travel agency gets their cut and you finally get what's left over.
When I look for a new assignment, I put out a feeler to several companies at the same time. For my last contract I was working with 4 agencies at the same time. Each agency has the same contracts for a hospital as the others and each agency has their "direct bill" contracts where the others don't have them. Some mid level management companies work with certain travel agencies and vice versa as some travel agencies don't want to get gouged by the percentage that mid level management companies take off the top of what the hospital pays out for a traveling RN.
Best advice is to find an agency that has health insurance from day one. Some agencies don't give you health insurance until the first day of the month after you start your contract. Also, the agency should be giving you a travel stipend, usually $400-500. Most agencies all offer the usual health insurance benefits, 401K, etc. Also make sure the agency is going to pay you the max rate for your meals and housing according to the GAO website for the zip code where the hospital is that you will be assigned to.
Being a travel RN isn't the glamourous lifestyle that people think it is. While it is a rewarding career and I enjoy it, bottom line is that travel agencies are in business to make money off of you. The less they pay you the more your agency recruiter is going to make in commissions. You would be surprised as to the difference once agency vs. another will have in regards to what they will pay you. Contact several agencies, don't let them know you are working with other agencies and see what they are going to pay you. Before going on to any agency website and signing up and filling out stuff make sure you get a quote on: travel stipend, hourly rate of pay (taxed), meals and per diems rate, cost of health insurance (is it day one) etc. You'll also be surprised how much one company vs. the next charges for health insurance. Some are reasonable, some are outrageous. If the agency isn't willing to let you know what the rate of pay for a certain assignment is, just contact another one.
Hope this helps and good luck.