As long as you can do the job right, there is no contest here. BMPCC4k all the way. The X-T3 is a nice mid-range photo camera, but it's a still camera first and a video camera second with all the limitations that entails. The BMPCC4K is a very solid entry level digital cinema camera. It lacks the more full featured functionality of it's bigger brothers like the BMPC or Ursa Mini, but it inherits from their roots.
Yes, the file sizes are large and you shouldn't underestimate the cost of data storage. If you are planning on shooting a feature length film in 4K RAW, expect to be using around 10-17TB of footage (I am working through post on a six episode TV series shot in 4K on a mix of Ursa Mini and BMPC4k currently.) What you get for that extra data is well worth the cost though, especially on a large project. You get a degree of flexibility in your color that is unmatched in the DSLR/Mirrorless camera video shooting world, which will greatly assist in your color grade efforts in post. Also make sure you get your hands on a color target to simplify fine tuning your color as you go and save yourself lots of time in post.
Do make sure you have sufficient lighting and know what you are doing when it comes to exposure though. While the Black Magic line of digital cinema cameras is a great line, it does not hold your hand. It expects you know what you are doing and doesn't do a great job of covering for your mistakes, even up in to the Ursa end of their product line. They can produce amazing images if you use them right, but used incorrectly, you'll have far better results out of the DSLR/Mirrorless world.
I have experience shooting video professionally with both a Canon 5D Mark iii (with Magic Lantern installed) and most of the Black Magic line of digital cinema cameras. There is no comparison between them when used properly. There are a few special cases I'd choose my 5D over my Ursa Mini, but they are few and far between (mostly low light shooting where it actually has to look like low light.)
The one thing that might be a gotcha for you would be that if you do weddings, the Black Magic line is really not well suited for weddings compared to the X-T3, but for music videos and feature film production where you can control the light to fit your needs vs the other way around, Black Magic wins hands down by a significant margin.
