When choosing a slim modern LED effect flame room heater, consider the following factors:

1. Heating Power: The heating power of the heater should match the size of the room you want to heat. Consider the square footage of the room and choose a heater with the appropriate heating power output.

2. Energy Efficiency: Look for a heater that is energy efficient and has a high Energy Star rating. This will help to reduce your energy costs and environmental impact.

3. Safety Features: Choose a heater that has safety features such as overheat protection and tip-over protection. These features can help to prevent accidents and ensure that the heater is safe to use.

4. Design: Look for a slim, modern design that complements your decor. Consider the color, shape, and finish of the heater, as well as its size and weight.
   

5. LED Effect Flame: Choose a heater with a realistic LED effect flame to add ambiance and warmth to your room. Look for a flame effect that is adjustable, with different flame intensity and color options.

6. Control Options: Choose a heater with control options that are easy to use and allow you to adjust the temperature and flame effect settings. Look for a heater with a remote control or a digital display.

7. Noise Level: Look for a heater that operates quietly, without making too much noise. This is especially important if you plan to use the heater in a bedroom or other quiet space.

8. Price: Slim modern LED effect flame room heaters can range in price from around $50 to $500 or more. Consider your budget and choose a heater that fits within your price range.

By considering these factors, you can choose a slim modern LED effect flame room heater that is energy efficient, safe, and complements your decor, while also providing warmth and ambiance to your room.