Jim, there are many ''right'' choices on the Rio and it''s a function in part of what you''re looking for and what you want to pay (altho'' they are all a bargain relative to U.S. and Caribbean standards).
While it''s subjective, we were looking for:
1. a marina vs. good anchorage, for safety (a wise choice IMO)
2. to be near Fronteras vs. down the river a ways, preferably a dink''s ride away from the town
dinghy dock, for ease of grocery shopping, so buying gas or LPG nearby was easy, etc.
3. staff who were accustomed to boats being unoccupied for a while, knew when a
line needed tending e.g., and this was part of the marina''s services
4. on-site laundry; no more buckets in the cockpit for a while
5. marinas were a rare thing for us so, since we were splurging, we wanted a nice ''feel'' to the place; friendly, clean, interesting place to be, etc.
We ended up staying at Tortugal (tortugalmarina@hotmail.com; www.tortugal.com), which is to starboard just the other side of Fronteras, has a bit of a European flavor and also appeals to the ''Naturalist Tourist'' types with some very nice but basic cabins & huts. It was almost new in early 2002 and offered an ''over the water'' restaurant and large, open but shaded balcony with BBQ''s for yachties. The one downside is that the med moor arrangement, somewhat common up there, coupled with weekend Guatemalan boat traffic on the River, can produce surge at the docks and so the marina''s stern mooring
line plus your stern
anchor line (recommended) need to be snugged up firmly. (They have bow planks for getting off the boat).
As I said, it''s a personal thing, but we really enjoyed the place. And BTW Tikal was beyond amazing and magical. Highly recommend hiking up the trail into the park in the early morning dark, climbing one of the temples to the top, and watching the sun come up over the forest below, out to the horizon and possibly shrouded in ground fog with the peaks of the temples sticking thru, while the birds and animals start to stir. An unparalleled experience. (Take a flashlight for the hike).
Good luck and enjoy!
Jack