I'm just back from Rio Dulce.
I recommend Nana Juana Marina for their very nice restaurant, bar and pool (next to Ram Marina).
As for buses to Guatemala
City, there are three operators. We chose Linea Dorada because they have less stops and take (in theory) 5 hours instead of 6 at a cost of Q130 (the others charge Q65). It took us 7 and a half hours as the road was blocked first by an accident
then by road works.
Make sure you go to Tikal (bus leaves from Frontera on the other side of the bridge. You can stay at Tikal Inn, conveniently located at the entrance of the park.
Enjoy, we loved it.