I have 5.5 days in Costa Rica in August. I was planning on going to Manuel Antonio but I have heard good things about Arenal. Several people have said it's a "must see" while we are there. I looked it up and it seems like Arenal is about a 4.5 hr drive from Manuel Antonio.
We already have to spend 3 hrs each way from the airport so I'm wondering if it is worth it to spent the extra 4.5 hrs to drive to Arenal with the amount of days I have there?
If so, how many days should I spend at each place?

With 5.5 days, I'd pick one or the other. Go for MA if you want to go to the beach and have really easy access to fairly tame wildlife; go to Arenal if you want natural hot springs, fantastic adventure activities, and to see the volcano (not currently producing lava). We've gone to both a few times and have enjoyed them. If you wanted to see two areas, you could spend a night or two in the central valley area and do things like Poas, the Doka Coffee Estate tour, the La Paz Waterfall Gardenss, etc.

Make that Gardens - sorry!

I loved both places- would hate to leave either one out. We flew to a small airport near MA and a driver picked us up, spent a few days at the beach, flew back to San Jose and then drove to Arenal.

sunbum has a good point. If you really wanted to see both areas, it's doable. I think of CR as a place that I'll go back to every year, so I don't feel the need to pack too many places in, but if this is the only time you'll go, you could just get a little taste of each place.