To the OP’s question, I would say that both versions are used and useful depending on the situation. Therefore the only concern should be if you intend to put Emblems on them. At that point, you really have to decide which version fits your playing style better and which parts of the special you could do without for a majority of the time. For example, I have 2x Rigard’s with costumes maxed and I prefer Rigard with the costume for most uses, so for the one that I put emblems on, I followed the talent path that suits the costume version better, and no it doesn’t match the normal version’s talent tree optimally at all. I’m good with that though because if I want to use the normal version, I still have the option.
Melandor, Rigard, and Boldtusk are better with the costume for the 4* Attack Buff Tournaments, whereas Kiril is better without it for that same tournament unless you’re using in of the others as well. I find that Rigard and Melandor both with costumes works great together, as does Bold and Kiril w/costumes work well together.
It’s really a choice based on your personal preference, your roster and therefore your needs, and what progression you’re at in the game. Kiril is an excellent option as a defense team tank and War defense team tank, but C-Kiril is a viable option too if it’s an Attack Buff war.
Overall, I’d suggest you level them all, level all their costumes as well, and then use them for a couple of weeks in actual gameplay so you can get a sense of which version suits you best. Then go from there if you intend to put emblems on them.