Yes, I use Qubicle as well as MagicaVoxel.

I think I would describe their strengths and weaknesses like this:

MagicaVoxel

• Less beginner friendly. All buttons are a single letter.
• More powerful tools for sculpting, once you learn how they work.
• Great for very organic looking things. You can stamp out saved models in a scene.
• A super sweet rendering engine for awesome screenshots.

Qubicle

• Easier to pick up and use right away
• Plane feature is awesome. Isolate a single layer to change the insides of models or start in 2d.
• Better support for selection/copy/paste/moving things around.
• Crucial support for splitting models up into sub-objects.

For the game I've mostly used Qubicle, since I need to break up and animate all the parts independently.

But MagicaVoxel is a lot of fun to play with! Here is a cat I made:

Suggestions with a custom made model you say? That sounds like a great first topic for the Fan Art forum! :P
And if you just want to donate your expertly crafted model to me in the hopes that I'll somehow include it in the game, post some images in the forum and send me an email. Can't promise you I'll use it, but if nothing else I'm always looking for inspiration. :)

One thing I've learned for this game is that the models need to have relatively thin limbs in order to support severing them easily.
The first non-humanoid walker looked like this, and was really difficult for beginners to cut down:

Hello, what engine do you use?