Since you say it's for a road trip and you're going in May, you won't have enough time to really learn all the ins and outs.
There are some terrific new ones out or soon out that have Voice recognition (for input),,,but that may be over the top for your purposes. At a min, do not get anything without Text to speech (spoken street names). A 4.3 display is extremely nice if you look up a lot of nearby points of interest. Map dates are also key--they cheapies (and I just bought another (#12 or 13?) last week) --the cheap ones will have map datum 2 to 3 years old. Even the Garmins and TomTom try and pawn off 2 year old data with some current models--discounting them slightly adn then throwing in a coupon so that you can buy a current set for THE SPECIAL LOW PRICE of $50 or some such---which is a total ripoff...BUT, the point is moot if you are going on interstates and to well established/larger cities (they don't change much over time).
Live traffic update options are nice, but then buy one with a lifetime subscr, not a mo-to-mo subsc.--it's not foolproof---traffic updates are instant. LAg time means you could just as well get stuck as not. MSN service is nice also, but not at all necessary for routing.
Most of the GPS hardware is pretty comparable nowadays (even a lot of the Mio's and other lower priced options use current/nearly current chip sets. The key is SOFTWARE (not maps--whcih all come from 2 companies)--its' how the device operates ---how does do it's routing, how does it allow for searches, how does it handle re-routing, and so forth. Trouble is, you really need to try 2 or 3 for a few days to appreciate the differences. Garmins are nice--I have 4. I also have mios, Navigons, and a Pharos and used TomTOm extensively. For an extra 40-50, I'd go with the Garmin (or TomTOm)...but they still prem price their better gear--and compared (on feature set) to the top line Mios and such, it doesn't justify it.
john m.