Which is better for office work?

Either one. Both run the same Core 7 150U with 32 GB DDR5, which is plenty for documents, spreadsheets and video calls. Pick by color.

Is the 819US faster than the 820US?

No. Both list the same Core 7 150U, 32 GB DDR5 and 1 TB NVMe SSD. Any speed difference would be noise.

Expect the same lifespan. The internals are the same class, and the single memory slot caps both at 32 GB, so neither has an upgrade edge.

Do either have a touchscreen?

No. Both are 27-inch non-touch all-in-ones.

Can I upgrade the RAM later?

Only within limits. Both list one memory slot and 32 GB maximum, so you would swap modules rather than add more.

Is either good for gaming?

No. Both rely on integrated Intel graphics, which suit casual and older titles but not modern games.

MSI Modern AM272P All-in-One vs MSI Modern AM272P All-in-One

These are near-identical 27-inch MSI all-in-ones. Both use the Intel Core 7 150U, 32 GB DDR5, a 1 TB NVMe SSD and a 1080p panel, so performance and build feel the same. The main split is color: the 820US ships white, the 819US ships black. Buy whichever color fits your desk; there is no real capability gap.

Choose the AM272P 1M-820US if you want the white chassis to match a light room or desk setup.
Choose the AM272P 1M-819US if you prefer a black chassis, which hides smudges and blends with dark peripherals.

Prices

MSI Modern AM272P All-in-One: $1,524.71 at Walmart (checked today)
MSI Modern AM272P All-in-One: $1,379.99 at Walmart (checked today)
Difference: the MSI Modern AM272P All-in-One is $144.72 cheaper than the MSI Modern AM272P All-in-One today