Killies genetic predisposition is for them to live in a shallow mud puddle, so I don't think growth stunting is an issue in your tank. They are one of the most suitable fish for a small tank - mine is 33L and I have about 15 of various sizes so your guys are still in a mansion with just the 2 of them. Many people have them in tanks about that big or smaller, and breeders will have more in there too.
I think they will be fine in either tank - presuming the other tank has a lid. One of my big fry actually jumped right out this evening and landed on the floor with a full tummy. Gave itself a shock and went to hide on top of the filter when I'd put it back in. At feeding time it can get quite competitive!
You have plants and stuff so there are hiding places if they require. Your call in the end, if you aren't sure then leave them be a bit longer.