I don't think it really matters much which one you get. EZ Pass, I-Pass, I-Zoom, they all work on the same toll roads. I ordered mine online. At the time, EZ Pass seemed to have the most hoops to jump through and the most extraneous crap to deal with to get it. Some states, online or in person, have monthly fees (Indiana $1 a month, Ohio $.75 a month, NY and NJ, $1 a month, PA is $6 a year) but it all depends on how you get them. Initial refundable deposits vary (Illinois is $10, Ohio is $3, others have none if it's set up as autoreplenish with a credit card).
I have the Illinois I-Pass set up with a $40 minimum balance (whenever it drops below $40, it automatically replenishes the account using my debit card with another $40). It's very easy to administer online, and you can print out or save a copy of the transactions for submitting for toll reimbursement. My only real recommendation is to make sure there is no monthly fee on whichever one you get. Many of the Service Plazas have them, and you can pretty much just walk in and get one.