There are multiple pros and cons to using clip-on ponytails, and whether you use them or not depends on what you are comfortable with and the effect you want.
Clip on ponytails:- Easier to deal with for detangling/storage, since they can be removed- Can help keep your wig on your head better if you clip them through the wig and into your hair- Typically cheaper and easier to find- Easier to style, no need to worry about sagging “bag hair” in the back- More versatile, in that the base wig can be nearly anything, you don’t need to worry about head size, etc.- Often fuller ponytails than with built-in ponytails because of the bulk of the clip- Often less realistic than a wig styled into ponytails because of the bulk of the clip/how the clip attaches (hair coming downward instead of out from the head)- Often less realistic than a wig styled into ponytails because the base wig is usually just a short wig and not a wig parted for pigtails (though this can be changed if you get a parted wig or modify your wig to have a part)
Styled ponytails:- Often more realistic because the hair appears to be coming out of the head rather than a separate piece- Typically better with shorter pigtails and thinner pigtails
Personally, I love the look of styled ponytails, since they look far more realistic. However, for a wig as long as Miku’s and with that style of ponytail (a lot of modern moe-type designs are drawn with the ponytails situated downwards rather than sticking out of the head), I would highly recommend clip-ons because you can get the volume you need, help keep the wig maintained easier, and help keep the wig on your head easier. It’ll also be much cheaper than trying to get a wig that is long enough to have styled-in ponytails that are long enough, since it’s much cheaper and easier to manufacture the clips!
What I would do is look into going for the best of both worlds, which is getting a base wig that is parted for twintails and then adding clips on top of that. It might take some styling work (you maybe have to part it, sew in wefts to cover the wig structure, and then stub the hair yourself), but it will have a more realistic look. If you don’t have confidence in your wig styling skills or don’t mind the look of a short wig that isn’t parted, then I would go with the clip-on ponytails. If the video tutorial of a wig being parted and having wefts sewn in for pigtails isn’t clear enough on this point, you can’t just part a regular wig into pigtails because the direction of the rows of hair (the wefts) is incorrect for that style, and it would show where the wefts are sewn to the base if you tried.
Alternatively, here’s a method of parting a wig to my clip-on ponytails look more realistic that doesn’t involve stubbing.
I would also recommend getting net covers for your ponytails, which become nearly invisible in photos but help keep your pigtails nice and neat and prevent tangling. Assist sells them in Miku color for this reason!
I hope that helps! Good luck :]