Aldi has better dairy products, apart from yogurt. Half and half or cream I particularly find to be shortdated at Lidl.
Deli meats are better at Lidl. Chicken is better at Lidl. Beef and pork are better at Aldi.
Aldi has better prices on junk food, Lidl has more variety, quality is equivalent.
Unsweetened breakfast cereals are better at Lidl. Sweetened are better at ALDI. Oatmeal is better at Aldi.
Aldi has better chocolate Candy, Lidl has better fruit candies. Lidl has better nuts, but more expensive. Aldi has better baking supplies. Lidl has a more extensive pantry selection, but I prefer prices at Aldi and quality is equivalent. Except marmalade. Lidl marmalade blows Aldi out of the water.
Aldi has better frozen junk food, otherwise Lidl frozen is superior.
Paper and plastic goods are better at Aldi. Health and beauty supplies are far superior at Lidl. Ditto for baby supplies. I also prefer their cleaning supplies, but rarely buy them so can’t make a firm comparison.
Fruit and veg generally better at Lidl, but not enough difference to matter. Aldi generally has better non sale prices. Lidl a better selection.
Fresh bakery is better at Lidl, prepackaged baked items (sandwich bread, bagels, etc) far superior at Aldi.
Source: once a week Aldi shopper for 20 years, once a week at Lidl since they opened.