Choosing the correct LED strip is a matter of choosing the correct one for the correct purpose rather than one being better than the other.  12V and 24V are the most commonly available options for low voltage LED strips.  24V has been found to be better if you need to run a longer length of strip while 12V is useful for circuits that run on 12V such as a motorhome.  

Voltage Drop and how it affects LED strip

The main factor that affects the difference between 12V and 24V LED strip lights is called voltage drop.  Voltage drop refers to the loss of electrical potential (voltage) as the current travels along the conductor (for LED strip lights this is the copper track).   This phenomena causes the LEDs further away from the LED source to receive less power which may make these LEDs appear dimmer than the rest or to be a slighter different colour.  24V higher volt systems are inherently more resistant to voltage drop than 12V lower voltage.

Comparison Table: 12V vs. 24V

When to Choose 12V

1. Choose if you require shorter precise lengths: As voltage drop is not as frequent for this strip in lengths smaller than 5 metres this is a good choice if you need to fit your LED strip into smaller spaces or if you need lots of smaller lengths.
2. You are integrating with existing low-voltage systems: 12V is the standard voltage of electricity for automotive, boats, and a lot of RV/camper setups.  Being able to use a 12V strip will simplify connecting these lights into these vehicles. 

When to Choose 24V 

1. If you require long, continuous runs: If you are looking to install 5-10 metres then you should look to use a 24V LED strip.  It is a better choice as the risk of voltage drop is lower. 
2. The installation is high-power/high-density: High-density LED strips (120+ LEDs per meter) or high-wattage strips will draw more current.  24V LED strip minimizes this high current.  This reduces heat which will contribute to improving the lifespan of the LEDs as overheating is one of the main reasons LED lights can fail.