Every year I grow sweet potato slips from saved sweet potatoes that I grew in the garden. I started a few years ago. My first slips came from store bought sweet potatoes and from Norma. She sent several varieties, but most didn't grow well here. Purple however grew prolifically. In fact is is the most prolific sweet potato that I've grown. In addition it is my favorite for taste. When you get seeds (or slips) from other gardeners, you never know if it will end up being a favorite, or something you try once and never grow again. There are a few other seeds I've gotten that are like Purple - prolific and tasty and just wonderful. Cherokee Trail of Tears was like that (which sadly I can't eat anymore). As was Michihili cabbage. Everyone's garden is different. What grows well in my garden here isn't the same as what grew well in my last garden which is only 15 minutes away. When you find something so good you always treasure it.
There are lots of ways to grow slips. I've only used one so can't compare. I put three toothpicks into the middle of a medium small saved sweet potato (in my storage bin I mark them with tape so I won't eat them). Then I suspend them in some water to grow roots and those all important slips.
Garnet is one that I got from the supermarket. Everyone says to get organic to start slips as they put growth inhibitors on them if they aren't. But these were originally conventionally grown and had no trouble sprouting. I've found over the years that Garnet takes about two months longer than Purple to break dormancy and start growing so I start them in January. However this year I tried starting them on a counter away form the window. The slightly warmer spot made them sprout very fast. Usually they are this size at the beginning of May. Hopefully I won't have trouble keeping them longer. Worse comes to worse and I can pot them up. But Garnet does grow stocky and slowly in the cold so I might just be fine.
Last year I also grew Beauregard. It was my least favorite of the sweet potatoes and sometimes doesn't grow well. Last year it was pretty pitiful since it couldn't handle the late cold spell we had. Many of the slips died. Garnet and Purple lived fine. The cold spell slowed them down, but didn't kill them. Where I live that is an important feature as late cold spells aren't all that uncommon. So this year I'm dropping Beauregard and I have just two varieties. Some year I might do another trial, but I'm pretty happy with the two varieties that I have.