Lego and Meccano are amongst the most reknown model construction systems for children of ages 6 to 14 and up to 16 years old. They drastically differ however since Lego bricks require no tools to be assembled while Meccano relies on the use of a screwdriver and a wrench to fix together the various parts with bolts and nuts.
If you're hesitating between them for your kid's birthday especially they both sell at incredibly high prices, here's how they compare:
The below creature was made out of a 13 year old boy's imagination out of various Lego parts. many being from the model pictured to the right. The relative easiness of dismantling and re-assembling models encourages kids to create their own robots, superheroes, vehicles, etc.
The various Lego parts and bricks of several models can end-up being mixed all together in compartment boxes and sorted by type, size, color, etc. before being re-assembled into any shape and model. The educational level of Lego is obviously increased in this case compared to the construction of the exact given model and its step-by-step instruction manual.
This miniature Eiffel tower requires a higher dexterity to be assembled compared to bricks construction as it involves holding a tool with each hand, handling small nuts, working on both sides of a model, making adjustments to the metal parts prior to tightening the bolts, etc.
Dismantling it for building another model out of free imagination could not be as obvious as in the case of bricks, but requires higher skills and a lot of willingness which is of a higher educational value. The method of assembly is similar or identical to real life construction activities.