Both process the same data to a high standard. The real differences are cost, how long each takes to learn, and how far each goes in expert hands - decided by what you shoot and how much evening you have.

Two programs come up whenever someone asks what to process astrophotos with, and they are not close cousins. PixInsight costs around €300 and is the deepest processing environment in the hobby. Siril is free, open source, and covers the whole ordinary workflow well.

This is written by people who make a third program, so take the recommendation with that in mind - but the comparison below is the one we would give a friend, and it sends plenty of people to Siril.

The short answer

Learn Siril if you are starting out, if budget matters, or if you want a complete workflow without months of study. It calibrates, registers, stacks, removes gradients, plate-solves, photometrically color-calibrates, stretches and denoises. For most people, most of the time, the result is limited by the data rather than by Siril.

Learn PixInsight if processing is part of the hobby for you rather than a toll to pay, if you shoot enough hours that the last ten percent matters, or if you need masking and pixel maths that nothing else offers.

Cost

Siril: free, GPL, no limits, no account.

PixInsight: a one-time licence, around €300, which carries a year of updates - the software keeps working after that, but further updates need renewing. Expensive next to free, cheap next to a filter wheel - the real cost is the learning time.

Learning curve

This is the difference that decides it for most people.

Siril gives you a script menu. “OSC_Preprocessing” takes a folder of lights, darks, flats and biases and returns a stacked image without you knowing what any stage did. From there you learn the manual steps one at a time.

PixInsight presents dozens of processes with dozens of parameters each, and no default path through them. The documented workflow - WBPP, then DBE, then SPCC, then a stretch, then noise reduction, then deconvolution with a mask - is a sequence you must learn before you get a picture you like, and it is normal for that to take months. People who have climbed it rarely go back; people who bounce off it usually bounce off hard.

What PixInsight genuinely does better

Masking. Applying a process to only part of an image - the nebula but not the stars, the core but not the outskirts - is where it is unmatched. Most of the “how did they do that” in a great image is a mask.
PixelMath. Arbitrary arithmetic across images. Narrowband palettes like Foraxx, custom star recombination, continuum subtraction - they are all PixelMath expressions.
Deconvolution and noise reduction. BlurXTerminator and NoiseXTerminator are PixInsight add-ons (paid, and separate) and they are genuinely ahead of what free tools do.
Repeatability. Process containers and icons let you save an exact workflow and apply it to the next session unchanged.

What Siril does better

Getting to a result on day one. The scripts are an honest on-ramp, and a beginner has a stacked image the first evening.
Being free, on everything. Windows, macOS, Linux.
Speed and size. It is light, it starts fast, and it does not need a powerful machine.
Plate solving and photometric color calibration, built in and free, which was a paid-tool feature not long ago.

Where they are level

Calibration, registration, stacking with sigma rejection, gradient removal and a first stretch. On an average session, a careful Siril image and a careful PixInsight image differ less than either differs from the same data processed badly. Most “PixInsight images look better” comparisons are really comparisons of experience.

What about planets?

Neither is a planetary tool. Planetary work is lucky imaging - grading thousands of video frames, multi-point alignment, wavelet sharpening - and that is AutoStakkert plus RegiStax, or one application that does both. If you shoot planets as well as deep sky, neither of these two is the whole answer.

An honest third option

If the problem you actually have is that the sequence is the obstacle - folders of unprocessed frames, a dozen tutorials open, no finished image - then the choice between two expert tools may be the wrong question. That is what Akastroid) is for: it measures your frames and decides the parameters, on Mac and Windows, and it does planetary as well. It has a lower ceiling than PixInsight and always will, and we say so on the comparison page. There is a Siril comparison too, written the same way.

Try the free tier before paying anything, and try Siril before paying for that too - it costs nothing to find out which one fits your evenings.

The short version

Siril if you are starting, or if free and complete beats deep. It is genuinely good.
PixInsight if you want the ceiling and will spend months reaching it. Masking and PixelMath are the real reasons.
They are far closer on an average night than the internet suggests; experience is the bigger variable.
Neither does planetary work. That is a different toolchain.

PixInsight vs Siril: Which Should You Actually Learn?