Probably purifying gets you more CP but im not sure how much candy you'd need for that

Purify.

If you want.

It costs 20 candy to purify i think

Yeah.

Personally though i dont think that you should purify it, since 20 candy will already get you to a pretty high cp and purifying it will only raise it by a bit. Also the next team go rocket event is coming soon so you can change its charged attack then, and if you dont purify it and do what i just said you also get to keep the shadow bonus

Keep it as it is dont purify

Dont do anything with it

Regigigiggigigugigigigigas is useless in pvp and raids so there ain’t no point in using it anyway