Assuming all else is equal, then I generally value river access over being on a hill.
River access gives you Watermill, Garden, and Hydro Plant. Also gives you a trade bonus with trade routes. Has a minor defensive bonus against melee attacks (from one side). You probably have more river tiles (for irrigated farms). And though not popular, you have Sacred Waters as a viable religious belief.
Hillside gives you an extra hammer every turn. You also have a higher combat strength, and better vantage point for ranged units that are garrisoned. Disadvantage is that you cannot have Windmills.
Rivers win over hills in most circumstances.
Maybe if you are next to aggressive AI's, then you will want the stronger defensive capabilities of a hill. Or if Austria, your Coffee House can be built even on a hill.