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I think the best advice I can give you is to taste the 2.
BerryBaby is trusted source on General Cooking
Cilantro is more flavorful and used in Chinese snd Mexican dishes. It has a distinct flavor that some people love and others detest.
When the plant turns to seed, those are coriander seeds, also used in recipes.
Parsley is a milder flavor. Used in soups, salads and as a garnish.
They are both members of the plant family umbelliferae, along with chervil, carrots, parsnips, celery, angelica and others. They share some cultural properties- cilantro is sometimes known as "Chinese Parsley", though it's a European native. As far as culinary properties, I would second MMH's advice.
If you don't want to purchase both to taste, just crush a leaf of each and smell them. You will get an idea of the difference. I think of cilantro as brighter and parsley more earthy. The flavor profiles are quite different despite the similarity in appearance.
Nobody has mentioned this, but some folks like me find the taste of cilantro particularly unpleasant. I find it tastes like soap, especially if there is a large amount of fresh cilantro in the dish. That is for the fresh leaf. The dried form is useful in cooking, if added near the end. I do not find it nearly as pungent in the dry form, and don't find the soapy taste present. It's not useful in a chimichurri sauce, or if you are cooking for a cilantro lover, but in stews and such it doesn't seem to make much difference if you use fresh or dry. The only way you can really know if you like the herb, and the taste difference between it and parsley is to actually taste them yourself. In great quantity, fresh parsley is also a surprise to lots of folks. It can take on a grassy quality, and be quite pronounced. Flat leaf parsley also tastes different than curly leaf varieties, and depending on how it was grown, it may not have much taste or it may be strongly flavored. The only way to know is to nibble.
Parsley and cilantro, together with most of the annual and biennial herbs (basil especially) develop an entirely different internal chemistry when they go into the flowering phase; it's not just a matter of flowers tasting funny, the entire plant changes. Most of them also grow quite rapidly in this phase. Strictly speaking the plant should be put out to pasture at this point, but they still get harvested and sold- I think this has more to do with flavor differences than growing conditions- soil chemistry, exposure etc can make a difference, but commercial growers generally are pretty uniform about that stuff.
They are different plants and taste differently