The difference could be that you might say ‘Let’s take a bus’ whether you are at or near the bus stop or not - you could be somewhere else planning your journey. ‘Let’s get on a bus’ is appropriate if you are already at the bus stop.

Both make sense, but I think you'll find "take a bus" more natural in most situations. "Take" suggests a choice of transportation. (1) We are planning a trip from New York to Chicago. Should we take a bus or a plane? (2) How do your children go to school? They take a bus. "Get on" gives a bit more emphasis to the actual act of entering the bus. "Get on the bus," the impatient driver said to the misbehaving children who were already late for school. In your example, both work, but if my concern was clarity, I'd choose, "Let's take a bus. It will be quicker." By the way, you'll also hear "take the bus" in English. It's also correct in many situations.

get on = subir take = pegar

The word "get" means almost whatever the speaker wants it to mean. So yes, there is no difference in meaning because the slippery word "get" means "take" if that's what the speaker wants it to mean. "Get" makes it easy to speak English. However, that ease comes at the price of clarity. Using "get", you can say it so many ways: "Let's get in (or into, onto, inside) this bus" "Let's get this bus". ("get" works all by itself!) "Let's get there by (or on, in, ...) this bus"