My suggestion would be comparing a classical symphony from the period of Haydn and Mozart with a symphonic poem.
There are some shared elements but overall distinct differences.
The latter was a natural progression of the Romantic period whereby free spirits like Franz Liszt sought to release themselves from the constraints of the strict sonata form symphony.

On the other side of the spectrum you could look at some fairly recent pieces. Why not look at a modern era 12-tone piece like Schoenberg’s Violin Concerto, and then compare it to a contemporary era piece like John Adams’ Violin Concerto. Analyze how atonality has evolved into a more flexible system for today’s composers.

- Romantic with early, free atonality. (Works by Berg, or the early works of Schoenberg for example).

- Another good one would be a fugue by Bach and a fugue by Beethoven, compare how they were similar and how they were different.

- Or compare a Baroque Italian overture, with either a Classical or Romantic overture.

- Compare Baroque dances (such as “Minuet & Trio” or “Gigue”) with any Classical or Romantic pieces that bear the same name. They would be greatly different in style, but you would find surprising similarities

I would suggest doing it on pieces that didn’t intend to follow suit from a predecessor.
Why not something from Schoenberg and Montiverdi or Bach… show the full swing around of musical innovators

I’m afraid the first answer wasn’t very helpful, choosing two works from the late Classical period written only 20 years apart.

I would suggest this:

Haydn: Symphony No 104 in D major (‘London’)(from 1796 in the Classcial period)

and a piece that looks wittily back at the Classical period:

Prokofiev: Symphony No 1 in D major Op 25 (‘Classical’)(from 1917)

That should be fun.

How about Symphony No. 40 by Mozart and Symphony No. 5 by Beethoven.