Vernier Caliper and Meter Ruler both are used for measuring length but have different purpose and roles. So to know which tool should be used you should understand the basic differences in these two tools.
Vernier Scale is the precise measuring device which can be used to measure length including outside dimensions, inside dimensions and depth with only a small margin on error. While Meter Rule is the most simple tool to measure objects with medium length.
The accuracy rates of both tools are different, Vernier Caliper is good up to an accuracy rate of .005mm which makes it the best option when measuring small lengths. On the other hand the meter rule has an accuracy of .25 mm but works more smoothly and can be handled without any prior skill.
Vernier Caliper contains jaws and screw clamp which can be used to measure the outside and inside dimensions of an object for which it is the best tool to do so. It can also measure the depth of an object but meter rule cannot be used to complete these measurements.
So if you are measuring something up to 50 cm or under 1 meter then Meter Rule should be used while vernier caliper should be used for precise and small measurements such as 0.05 cm.
To conclude both tools, vernier caliper and meter rule are used to measure but the difference is the preciseness, accuracy and the things that these tools can measure. If you want to measure length, depth outer & inside dimension then vernier caliper is the tool while for measuring simple lengths meter rule is the tool which should be used.