I support young Tiki because she's more iconic to me, and also because I like the contrast between a little girl and a large, powerful dragon.
Added.
There were a few ideas I had for a down special/counter which borrow ideas from my Chrom/Robin duo and Hong's moveset.
I do like Hong's envisionment of Counter utilizing a flower and hitting both sides so all iterations stated will involve that.
Borrowing from the Chrom/Robin moveset, Tiki would activate counter and upon successfully countering an attack she would be able to transform to dragon/humanoid form depending on which form she performed counter. This would be the only way she could transform when compared to Zelda/Pokemon Trainer who can switch characters on the fly.
Another idea could be to tie the transformation to a taunt, this frees up moveslots for characters like Zelda/Sheik as well as Pokemon Trainer.
Though I personally think a combination of the two would be better but to exclude Tiki from the second one, this way it makes her transformation a little more unique.
In addition to this I would like to bring up a mechanic I mentioned a long time ago about stock matches in which you can choose your roster.
For example, if I play a three stock match I can lead with Fox while having Ridley and Little Mac on reserve, the only way to switch is for a character to be KO-ed. Now why does this matter?
Well if we utilize a moveset in which Tiki's transform is not tied to her Final Smash this brings up a lot of counter-play options if the opponents she faces are changing.
For one match-up it might be advantageous for her to fight Fox in humanoid form as opposed to her slower, larger, and more easily comboed dragon form. Likewise there may be match-ups in which her dragon form will be preferred over her humanoid form, for instance if her dragon form has an incredible aerial game it might be a stronger match up against Little Mac who supposedly has a medicore aerial game. This brings more of an incentive to choose both/all forms as opposed to sticking to one for any character that can transform/switch.