Patterns vs. Kits
Patterns
Patterns can come in a few different forms: digitally (perhaps ordered online on an Etsy store), in a leaflet (a small book, usually no more than 20 pages, filled with patterns of a similar theme), magazines, or a physical copy of a single pattern which can be found at most craft stores.
Pros:
Great for storage! Some are only a page of paper thick.
Often times cheaper than kits.
Can be wirelessly delivered to you as soon as you want it.
More personalized to your interest.
More likely to support a small business, especially when purchasing on ETSY.
Cons:
You only get the pattern - you have to buy the fabric and floss separately if you don't already own them.
Can be very expensive if it is a limited edition pattern or a pattern no longer in print.
Kits
There are a few major kit producers: Dimensions, Bucilla, and Janlynn, with some smaller companies also floating around. A lot of online stores of also started packaging their patterns as kits to sell them.
Dimensions, Bothy, Bucilla, Mill Hill (most kits involve beading), Janlynn, Pine Mountain Designs, Etsy
Pros:
Everything you need is in one place!
Excellent for beginners.
Sometimes come with a frame for finishing.
Cons:
Can be expensive depending on the level of expertise and size of the kit.
Often times the thread and fabric included are not great quality.
Especially if buying from a big designer, there will be less tailored kits to your specific interests.