Learn Finnish: Minkälainen akrobaatti lavalla hyppää? - What kind of acrobat is jumping on the stage?

They both mean "what kind of" and are basically interchangeable in Finnish. millainen is slightly shorter and very common in everyday speech, but you can confidently use either one. Because they act as adjectives, both will change to match the noun they describe.

Does minkälainen change if we are asking about multiple acrobats?

Yes! Just like regular adjectives, minkälainen has to match its noun in both number and case. If you wanted to ask "What kind of acrobats", you would use the plural form minkälaiset to match the plural noun akrobaatit.

Why is the verb hyppää at the very end of the sentence?

Finnish word order is quite flexible. Saying Minkälainen akrobaatti hyppää lavalla? is also perfectly correct and neutral. Putting the location lavalla before the verb hyppää just slightly shifts the emphasis, drawing a bit more attention to the stage itself.

Why do we use the -lla ending for lavalla (on the stage) instead of -ssa?

lava means stage, platform, or pallet. Because the acrobat is performing on top of the open surface, we use the adessive case -lla (on). If we used the inessive case -ssa (in), it would sound like the acrobat is somehow physically inside the floorboards of the stage!