Tortuguero is primarily a canal-based environment, so you'd be doing most of your touring via boat and probably some walking on the beach to see the sea turtles. Corcovado would be more hiking in a jungly environment. I'd look at which of those appeals to you more.

Also, you can swim in the ocean in Corcovado. It depends on where you'll be staying and doing activities.

If you choose Tortuguero, the trip over in a boat is half the fun. As vc said, most of your touring will be done this way. The point where the river meets the sea is very cool, and you're sure to spot wildlife. Or you could boat in and fly out or vice versa. 2-3 nights would be a perfect length of time for this trip. The Watsons are known for leading a quality tour here.

I love, love,love Corcovado and the general area for its remoteness, but I like to stay closer to a week in this area.

The two are very different. A lot of wildlife in both parks. But being in Corcovado NP means one has to overnight at La Sirena Ranger Station. And logistically coming to it is much harder then getting to the heart of Tortuguero NP, the tiny hamlet of the same name. In my opinion, if only for 2-3 nights, then go to Tortuguero. If having more then Corcovado is a place to go for wildlife. And swimming is not the activity you want to waste your precious little time.

Check out the aguilla de OSA Eco lodge. Just up the coast from the Corcovado entrance.

The Osa can be pretty rainey in late July/August

Thank you all so much for the info. I can now consider the differences between the two. With only 2-3 days it sounds like tortuguero would be my best option. May have to save the other for a later trip. Traveling through the canals sound very interesting. Can't wait to see all of the wildlife!

It is really interesting. We boated in and then flew out on Sansa. We saw a lot of wildlife in the area - lots of blue morphos, monkeys, cool birds, poison dart frogs, crocs, otters, and caimans.

Is Tortuga Lodge a good place to stay? Or do you recommend any others. Looks like Tortuga Lodge is all-inclusive, which would make it easy.

Yes, it's an excellent place to stay. We picked it based on the reviews, and we loved it. Some places put you into groups and have you eat your meals with them, tour with them, etc. but Tortuga Lodge let us do everything on our own which we loved. They have a reputation for good food and service, and they really lived up to it. They're slightly pricier than other lodges, but definitely worth it for us.

Can you do both??? I know I'm late to the party but...we took a tour with one company that included the Arenal and Manual Antonio areas. Then we also hired the company that owns TL to arrange a 1 night trip to Torteguero, since it wasn't included in the other trip. If I go back I will use that company (Costa Rica Expeditions) to do the whole trip and I would go to Corcovado too, I still regret we did not get to go there! I would skip MA next time completely and just try to get both Corcovado and Torteguero in one trip, but I am a person who loves wildlife!
We stayed at the Tortuga Lodge, only one night, and it was incredible, and it wasn't enough. The location, guiding and food were all excellent. Be aware it is not air conditioned, but we found it amazing sleeping listening to the crickets and in the morning awaking to the sounds of the howler monkeys. If you search my screen name you will see my review from 2007.

Tortuga Lodge is sounding great already! I can't wait to go. I will probably fit in COrcovado on my next trip. This summer will be my third trip to Costa Rica and the place has really stuck on me. Short flight and incredible sights so I just want to keep going back. I spent the day at Tabacon hot springs a couple of months ago and would love to return there also!
Thanks for all of the advice.