Second seat for 3 1/2 yr old: booster or car seat?

[campbelk]

I'm looking for a seat for my son to use in our second car next Fall. He'll be 4 in January 2007 and I need a seat to use while I drive him to preschool ~30 minutes away. Our nanny will pick him up using our other car. I currently have a Marathon for him and a Roundabout for our daughter (currently 9 months). He weighs 39 lbs now and we're in the second from the top slots in the Marathon. Should I get a velt positioning booster seat such as the Britax Parkway for the drive to school? What are my options for a 5 point harness that's not quite as expensive as the Marathon. This is for a 30 minute ride twice a week so while safety is a big concern, ease of use/size/etc. is not. The seat will stay installed in our 4Runner (no LATCH). I love reading this board for all of the useful info.

Thanks,
Keri

[Joolsplus2]

How about a Cosco Apex (might also be called Alpha Elite)? That harnesses to 65 pounds with tall slots and becomes a good booster later, and only costs $129ish and Coscos are known to install tightly and have nice non-twist straps (nowadays... they used to be more like cooked spaghetti, lol).
Julie CPS Tech and mom to 3 in seats
http://www.cpsafety.com/articles/RFAlbum/SarahMA.aspx