Here is my perspective from owning and carrying both guns on and off duty for many years.
Yes the G19 is considered by most the standard to measure others against. The 19, is an excellent pistol, it is dead reliable, accurate, especially the Gen5, or mine have been and is simple to maintain, repair and upkeep. It does however have it faults, but there are caveats to them.
The ergos are by far not the best out there, this is however usually measured in a static environment, gun shop, show etc. Almost every other pistol out there feels better when your just holding it and finger banging it. The Glocks ergo go out the window when you start running it hard, such as in a high stress engagement. When time and hits on target are the purpose, the 19 and other Glocks excel at this, ergos be damned.
The Glock trigger, again, not the best by any stretch of the imagination, but once broke in, it isn't all that bad. It isn't a 1911 but it also isn't prone to breakage or failures. The one part that can break, trigger return spring, doesn't render the pistol useless in the event it does give out. You can still manually move the trigger forward and stay in the fight. The 320 on the other hand, if it's trigger return spring fails, you will need to throw it at the target, cut the pistol is now useless.
Aftermarket support and parts availability have no equal when it comes to Glock, and I am a 320 armorer, and some of the 320's parts are like hens teeth. It is easy to locate just about anything you need for a Glock, OEM or otherwise, and it takes no time at all to get a broken one up and running again.
Now for the 320, X, Pro or any other series of 320. They are fantastic weapons, and I do like the 320 quite a bit. I have owned several since 2015, and they have all been dead reliable.
The 320's ergo are very good even with the standard frames, and are even better with an AXG, Wilson Combat or any number of other manufacturers. This is a huge plus for the 320 and where the modularity pays off. Again this is when holding one in a static environment, when you run the 320 hard, the high bore axis becomes more apparent. Now I'm not saying it's the end all be all and will cause you to have issues shooting it fast, but it is something that will be observed, especially in the smaller models, the X compact, and X carry, and pro carry models. In my experience even though they have better ergos, it takes just a bit more to control them under rapid shot strings. It's just physics, and there ain't much that can be done about that.
When it comes to deliberate slow to medium speed shooting, the 320 and it's trigger is awesome. The 320 trigger is far more forgiving due to its single action characteristics. Then again I can do the same with my Glock, but I have been shooting them for years. The 320 allows you to do it without much work.
Parts with the 320 are a pain in the arse to get. I have had trouble getting guide rods and a few other things for my department, and again I'm an armorer. Aftermarket support as far as parts is getting there for the 320, and I suspect will continue to get better as time goes on.
One thing that has bugged me about Sig and the 320 since its introduction is their QC sucks, plain and simple. I have a local LE distributor I purchase my 320s from so I get to handle all of his stock when picking one out. The last time I purchased one I looked at 7, 320 pro carrys. Every single one of them had some sort of issue, optic plates finishes looked like they had been tumbled with rocks, rear sight channels not cut evenly, triggers all over the place as far as break and reset, one had a dead front night sight, one had FCU that wasn't machined correctly, the trigger bar rubbed on it during the pull increasing the weight to the point of almost not breaking, it goes on an on. I typically have to pick the one that has the least amount of issue and move on, that chaps me. It just says to me they are building as fast as they can and damned everything else. Volume over quality, and honestly, with who is the CEO Im not surprised. he did the same thing with Kimber. FWIW I would never purchase a 320 without first putting my hands on it and inspecting it. Glock have had some QC issues as well, but they aren't typically as obvious as Sigs.
I digress, Either one of them will likely serve you well and do whatever you want it to. The X compact is a fine pistol and the one I had was a great shooter. The Gen5 G19 I have is also a great shooter and has been dead reliable. Ultimately you need to decide which one you like best. If you can find a range that has the two for rent, I would start with that. Don't just poke hols in paper with it either. Do some strings of fire if possible, so you can really get the feel of each one.
Sorry for the long post, and I wish I could have helped a bit more. The next question is what is in my holster right now?. Well currently I am carrying My Gen5 19. Despite Sig fixing the drop fir issue on the 320's they haven't put enough distance between them and that for me to trust the 320 100%. With that, I may also carry one of my 320s again next week on duty, who knows.
TXPO