They’re all good and they all do something different. I tend to prioritise 4* because they can be up and running faster and cheaper, and a lot of people consider the piggy to be an honorary 5* (as indeed is costumed Rigard). However, don’t take my word for it, there are plenty of opinions on the forum and they won’t all agree. In the end it’s your game and if an elemental defence down sniper is what you want/need right now, go for it. Don’t leave Gulli on the sidelines though; he is very useful.
I found it quite amusing how everyone was saying “don’t worry about Leonidas, you can always get him back later” etc, as indeed I would have done, and then it turned into “ah - you have his costume? MAX HIM NOW! Possibly two!” Not all costumes make that much difference, but Leo’s does.