I own both. They are definitely different and a different experience.

I owned a FN Hi-Power in the 1970s and had used a 1911, P-38, and previously owned a S&W 28-2. Familiarity with a P-35 type is why I chose to get the SA-35. I didn't want the newer FN manufacturing or their large design revisions in the latest. That and the decades of governments with positive experience with the P-35 type. Browning's cam lock design is extremely reliable with feed in the P-35 types. I like it a lot compared to Browning's earlier barrel link design in the M1911.

I got a chance to hold and inspect a new SA-35 and that sealed the deal. Very well made. No frills all business. Plus some changes everyone who had the P-35 FN really wanted them to make -- rounded hammer spur and removal of the magazine disconnect that could hang a magazine drop. Very nicely made. Everything that matters is right. Forged slide and frame unlike alternatives with cast frames. Stainless match barrel. Nice barrel crown.

A CZ 75 nowadays has a cast frame. Well made and reliable like the SA-35 although the CZ has some periodic maintenance things every few thousand rounds. It is a nice DA/SA P-35 derivative. I prefer the forged frame.

I got the M9 to get a 90 series 92 type with F & S variant. I wanted a tried and true DA/SA to go with the SA only SA-35. The M9 doesn't have the back strap recurve and fits my larger hand better and recoil control better than the 92FS. Other than that, there are no significant differences. It is no frills all business. The new civilian M9's have a dovetailed rear sight unlike the early civilian and all the US military ones. The M9 has an aluminum frame and forged steel barrel. You must attend to slide/frame lubrication to keep that dissimilar metals rail sliding in good condition. Also, I had some brief experience with Walther's locking block design in the P-38. That design is what Beretta chose for the 90 series. One feature of that design is no barrel tipping. It is a very reliable design and I like it a lot.

Like the P-35 types, it has decades of governments with positive experiences with the 92 types.

What is different? They weigh almost the same. They have the same magazine capacities.

The SA-35 is a bit smaller in every dimension. It feels smaller than the M9 more than the actual size difference. The SA-35 is more concealable than the M9 due to its thinner size. That's one notable difference.

A significant difference is that the SA-35 is SA only whilst the M9 is DA/SA. That said, the M9 DA trigger pull is quite heavy -- 12 lbs -- and a l-o-n-g travel to the sear release break. The M9 SA pulls is about 6 lbs and has some not minimal travel to the wall before the sear break. The SA-35 is SA only, the trigger resets in a short travel, and has a short travel to it's wall before the sear break. SA pulls didn't feel significantly different to me although I didn't measure them. I just felt them.

I like them both very much. My M9 is a household weapon. My SA-35 is a smaller than M9 full size carry weapon. I have a Walther if I really want concealed small.

I don't think you can go wrong with either as a reliable well-made weapon. The differences I noted are things to consider about what is right for you. If you want an M9 with a back strap recurve, the 92FS instead of the M9.

They both shoot very well.

Hope this helps.