Having been to both hotels and recently been to La Siesta, I would say the Bitacora wins hands down. The Siesta is nicely decorated and the rooms are nice, but we did have a few cockroaches in our room (YUK)
This never happened at the Bitacora. I found the pool area to be not as clean as I had hoped there too. I know people say the rooms are dated at the Bitacora but I think cleanliness there was second to none.

The pool bar at La Siesta is very expensive with a coke costing 3euro40. And I found the menu to be not as varied as the Bitacora. The food in the restaurant doesnt even compare to the Bitacoras. They have themed nights 3 times a week and i found the food to be horrible (example one of the themed nights was serving tripe ) The dessert menu doesnt hold a candle to the Bitacoras with only a few gateaux and ice cream to choose from. If you are travelling with children please not that La Siesta doesnt have a childrens menu.

I personally would not go there again, I felt much more relaxed at the Bitacora - had a really good time there.