I made an earlier post where I mentioned that I use ASTAP to stack due to it's speed. It's a little more than that so here's why/when I use ASTAP instead of Siril for stacking.
When I started doing astrophotography some 15 months ago I started with Siril because I didn't want to spend any money until I knew what I was doing, and because I could just use the provided scripts to get good results without having to think much. Also, Siril has good tools for stretching and processing so I can stay in the one tool and do everything but final cleanup where I use Pixelmator as I'm on a Mac.
I had been using ASTAP to solve and continue to do so, but it also stacks, and after a recent update in September it has been stacking well enough that I feel comfortable using it instead of Siril to do most stacking, especially on large projects. The way ASTAP uses resources is far different than Siril where large amounts of RAM, Disk and CPU are consumed. If you have it Siril will use it. It's not uncommon for Siril to create 100GB+ of process files for large projects. To keep things speedy I tell Siril to use all my CPUs (which is the default) and it will allocate 20GB+ of RAM on my 32GB machine.
ASTAP on the other hand will only use one CPU, creates only one master file for flats/dark flats, one for darks, and then the final result. If I use my Nikon camera, it will convert the RAW NEF files to FITS, but it still consumes far less disk space during stacking. In addition, for a job of 200 to 600 exposures (typical for my larger sessions) ASTAP will complete stacking in half the time or faster.
The results are not similar, i.e. the noise looks different, but the desired data is there for both apps, and after processing it's hard to tell which program I used to stack. Regardless of which app I use to stack I still use Siril to do the stretching and cleanup. Also, for Moon shots, or for when I want to pull out the Ha and OIII data from my OSC shots I still use Siril to stack.