Would you recommend Lumix G9 or Sony a6400?

Lumix G9 is the better

g9 , its has better pixel rating than the sony .

The Sony. But it's mirrorless 35mm sensor cameras that are trending now. The image quality is so much better. Yes, you can still use them on auto like and P&S, but at least you have the potential to do better.

Cameras are tools.  As with many tools, say, a hammer, there are a variety of cameras each designed for specific end users with specific needs, expectations and budgets.  Without knowing what types of photos you need or want to make, it's hard to say which one is best for you.  I mean, a Ferrari is an amazing vehicle, but if you're a pig farmer who needs to haul pig manure, you'll take a pickup truck over a Ferrari La Ferrari any day of the week.

In terms of technical specifications and capabilities, the A6400 is the superior camera in many ways.  First, the A6400 has a larger APS-C sensor (crop factor of 1.6x) vs the smaller Micro 4/3 (crop factor 2x).  In order to fit 20MP into a small sensor, the pixels have got to be smaller.  The smaller the pixel, the more noise you'll get, especially at higher ISO settings.  The Sony A6400 has at least a full 1 stop advantage over the G9.  You can compare ISO performance of these 2 cameras here: https://www.dpreview.com/reviews/image-comparison?...

The Sony A6400 has a higher maximum ISO setting than the G9, but don't let that concern you.  Why?  Because at some point (much lower than max ISO) the image quality gets too low anyway.  It's like having a car that has a top speed of 200 MPH, but the car becomes undrivable at 100MPH.

The difference in resolution is a modest 4MP (20%).  If you think you'll be doing a lot of tight cropping or larger prints, then 20% can be an important thing for you.

Bokeh and blurring backgrounds is a big deal in portrait photography.  Because the G9 uses a smaller sensor with crop factor of 2x, the lenses are 1/2 the focal length compared to a full-frame cameras.  However, the depth of field produced is the same as closing down the full-frame lens by 2 stops.  In other words, a 50mm lens on the G9, for example, shot at f/2.8 produces the same angle of view at a 100mm lens on a full-frame camera shot at f/5.6.  Thus in order to get the same depth of field, you'd need to shoot at (in this example) at f/1.4.  There are lenses that will allow you to do this, however they're prime lenses.  As for the quality of bokeh, this is due to the optical design at that of the aperture, thus the camera can't have any influence on bokeh.

A huge advantage for the G9 over the A6400 is that it has in-body image stabilization (IBIS) of up to 6.5 stops!  If you need to shoot hand held in low light, then this could be the main reason to go with the G9 over the A6400.  Even though the G9 has 1 stop poorer ISO performance, the 6.5 stops of IBIS completely negates that, providing your photographing something that is not moving so fast that you get motion blur.  Six and a half stops is the difference between having to shot at 6,400 (with the A6400 without a stabilized lens) and ISO 100 (lowest ISO setting) with the G9.  The image quality here is tremendous.  Yes, of course there are plenty of lenses available for the A6400 that have stabilization.  But with IBIS, ALL lenses are stabilized, even vintage manual focusing lenses from the 50s, 60s because the body is doing the stabilization and not the lens.  If you're a low-light shooter that routinely can't or don't want to use a tripod, then I'd just might go with the G9.

The A6400 is a better camera for those shooting with flash.  Unlike the G9 which has  a max flash sync speed of 1/250th (which is great), the A6400 has high-speed sync allowing the use of any shutter speed just as long you're using a capable flash, too.  This can be critical for those shooting outside with a flash where the flash is used as a fill with larger apertures.  Portrait photographers will greatly benefit from this.

The G9 can shoot 20 fps (83 RAW images before the buffer is full) compared to just 11fps (buffer capacity: 42 RAW images) with the A6400.  This makes the G9 a better camera for sports providing that its AF tracking is up to par with the A6400.  If not, then having that many more fps could often been useless if you're getting an extra 10 blurry frames per second.  If you're a sports shooter, I would STRONGLY recommend a DSLR instead of one of these two bodies.  Or, go with a higher-end mirrorless like a Canon R6 or Sony A9.  The Sony A6400 has gotten pretty good reviews regarding its AF tracking capabilities compared to older Axxx models.  I just don't know how well it compares to the G9.

I would read the reviews of these two cameras at imaging-resource.com.  IR goes into detail about the real-life shooting performance of these two cameras.  This is something a spec sheet like this one: https://www.dpreview.com/products/compare/side-by-... can't show.

Don't forget to look at the various lenses native and adaptable lenses available for these two models.  You may find that one has the ability to use a specific lens that you want/need to use that the other body does not.  Adapting lenses with the G9 can be more advantageous if you use the $300+ Metabones Speed Booster.  This narrows the image circle down to M43 thereby increasing the illumination by a full stop.  Thus, an f/2.8 lens, for example, becomes an f/2 (note that the depth of field doesn't change, just the illumination).  Downside is that at around $300 USD, the MB SB adapters are expensive.

No, I'd buy a DSLR.

Yes but I would not buy either one of them personally.  I do not like mirrorless cameras but if someone else want them, they are free to buy them. The Sony has a larger size sensor and so it should give you better image quality. For people who want better image quality than these 2, I would recommend a Full Frame camera instead.