In U.S. English, "package" would be better. "Pack" can be used simply as a short form of the word "package," but it is most often used in a few specific phrases, notably "a pack of cigarettes," "a pack of cards," and "a six-pack of beer." Pack usually refers to a small package, but it is usually used for familiar, standardized packages of a known, customary size. For example, if you say "a pack of cigarettes" everyone thinks of a package of 20 cigarettes. For things that are packaged in many different sizes, so that you have to name the size, "pack" sounds odd and "package" sounds natural. People enjoy collecting obscure examples of collective nouns for animals in English ("a pride of lions," etc.) but most of them are rarely used. However, it occurs to me that we do have some fairly specific collective nouns for packages and packaging of commercial products. "A book of matches," "a case of beer" (24 bottles), "a ream of paper" (500 sheets),

In the US, "package" is used for most items that come in soft packaging (e.g., cellophane, soft plastic, paper). Pasta is also sold in a cardboard box so it is also appropriate to refer to a "box of spaghetti." However, if the pasta is wrapped in cellophane or soft plastic, then "package of spaghetti" is appropriate. Similarly, rice can be sold in a box, a plastic bag or a canvas sack. Thus, one could refer to a "package of rice," "box of rice," "bag of rice," or a "sack of rice." "Package" can also refer to anything that one receives by mail that is not a letter, regardless of whether it comes in soft packaging or a cardboard box and regardless of size. For example, the postal systems handles a lot of "packages" around Christmas. If an item is small, one would often refer to a "packet." Starbucks sells individually wrapped "packets of instant coffee." Yet it is appropriate to refer to a "pack" or "packet" of gum. A "pack" can also refer to animate objects: a "pack of wolves," a "pack of dogs." In short, the use of "pack," "package" and "packet" is highly variable. :)

I would say a "500-gram pack of spaghetti". The second sentence is correct, but a bit awkward to say because it's so long. Pack and package can both be used. But "pack" is often used when talking about groceries, I guess because it's quicker to say.