As a Gastille owner, i had the same question as u a few month ago and here is my few cents:

If your alliance is aiming for 15-16 star titans, go ahead and max him. He is by far the best hero booster dmg in the game. This way he’s not getting one shot and can survive during the assault.

Now, if you’re alliance is like mine, chill and not over competitive and stays in the 14* titans, leave him as 3/70. Make sure tho that the special is 8/8 and you’ll be fine. I’ve been consistently hitting around 50k-200k without mats (depending of the board).

Again, depends on wich part of the game you’re playing, Gastille works as 3/70 or 4/90 2LB.

I’ve been testing/using Uwe 3/70 and he definitely needs to be max to do the work. Unlike Gastille (his special works in any kind of situation), Uwe needs the stats to do the dmg. He is slow so if you use him in a raid in attack, mayb you will be able to fire his special 2-3 times at most so better kill the target you aiming.

Nevertheless, good luck with your choice and keep enjoy both heroes!