We've just visited on our own but here are 2 companies that get great reviews on Trip Advisor:
http://www.edventuretours.com.mx/
http://alltournative.com/

My photos with blog and travelogue links on the main page of each collection are below in case you're interested. Happy trails!
http://www.flickr.com/photos/staceyholeman/collections

TA forums are largely controlled by destination tourism or real estate commercial interests so take the reviews on TA with a grain of salt.

We did the eadventure tour when we went to ClubMed Cancun. It was well run and had a nice schedule. Our tour guide spoke everything in 3 languages for our mixed group (11 people). We went to both Tolum and Coba. It was a LOT of driving but we thought it was well worth it to see more of Mexico than the tourist areas. I would do it again.

I can vouch for Edventures. We have used them for several tours in the area and each time they have been responsive (when booking online and righ tbefore the tours), professional, and have good tour guides. They are reliable and a good value in an area overrun w/many tour guides and tour companies.

We have a condo in Playa del Carmen and when renters ask for tour company recs we rec Edventures first due to our own experiences w/them. After many such referrals, we, and our condo manager, have had no complaints and have had many good reviews of them from our renters, some of whom have gone to Tulum and Coba with them.

We had a great time with Edventures (though be advised they're more oriented towards snorkeling than towards archaeology).

Honestly though, you could probably do both sites self-guided if you have your own vehicle. There are also guides at the entrance to each site you can hire on the spot.

If you decide to drive yourselves, consider hitting them separate days so you can go 1st thing in the morning. For example, when we arrived at Tulúm as they opened we had the spot in the parking lot closest to the entrance and practically had the site to ourselves. By the time we left there were >20 huge buses in the parking lot and it was really heating up (June).

Thanks to all for the great suggestions! If I Did rent a car - what is the driving time to each and the entrance fees to the ruins? I Am staying at Excellent Riveria Cancun and how do you find out if you can still climb Coba? Or just stick with a tour?

You're probably about 90-minutes from Tulum and about 2 hrs 15 minutes from Coba. Given that, I'd try to do them both in the same day--Coba in the morning and Tulum in the afternoon. Crowds are actually worse at Coba in my opinion because everyone is on the same narrow paths whereas at Tulum it's more of an open layout.

We climbed Coba. It was just this past October. I assume anyone who pays to see it can climb it.

I would not rent a car. I'm a pretty adventurous driver but those roads were narrow and not very well marked and we felt like we were driving to the ends of the earth. If I'd had to navigate that myself it would definitely have impacted my enjoyment of the trip.

The above poster has a good idea of time frame and the crowd factor, although the crowds were not a huge hindrance to enjoying things.

Also the tour guide was a fountain of information. I really thought they did a great job.

Thanks for weighing in, Bob, but do you have a preference? I'm leaning towards green at the moment.