Generally there is an inch or two of more leg room on the upper deck. The cabin is smaller so fewer passengers to share facilities and cabin crew with. Usually the emergency row seats upstairs have quite a bit more leg room and are probably the most desirable economy seats on the plane.

The window seats in the middle of the emergency row upstairs seem to have more of a feeling of privacy. There is plenty of room to exit the row from the window seats without climbing over anyone. All the window seats upstairs feel like they are more roomy because there is a wide ledge along the windows with small storage bins.

The actual bulkhead seats at the front of the cabin on the right side (when facing toward the front) can be a little more cramped depending on how close the bulkhead is placed.

There are a couple of downsides you should also be aware of. If you are traveling heavily laden, you might prefer to wheel your carryon straight down the aisle, rather than lugging all your belongings up the narrow, steep stairs to the upper cabin. In addition, storage space upstairs is more limited. The overhead bins are smaller and generally won't accomodate a full-size carryon. However, there is usually a closet at the top of the stairs and flight attendants will usually help you store your large piece there.

If you have a window seat you will have additional storage between your seat and the window for smaller items you might want access to during your flight.

If it were me, I would almost surely make the change to upstairs.