Ive got a 338 stiller action. It is a beast. Long and strong. I got it on a deal is why I chose stiller over anything else. Thing is smoking accurate. 338LM
Don't worry about either manufacture, they both are reputable and both will perform just fine and I don't think you will be able to tell the difference.
If they were the same price, I would flip a coin. Unless the Defiance has an integral rail. Ive always liked that feature.
Unless the defiance isn't as long as the stiller. I like the extra meat.
If the stiller is considerably cheaper, boom go for it. I honestly don't know what the prices are right now.
other advice is get the bottom metal, I think Seekins BM iirc, that allows you to load to a longer oal. Really helps for bullet selection and chasing lands...not that anyone actually shoots out a 338 barrel. lol.
Good luck,
DT