Two of the Holosun standaout models, relative to their other models, are Holosun 508T and Holosun 509T. Both of them for shooters seeking robust performance and versatility. In this article I will compare them, will let you know which features they have, and generally some useful info that can help you to make up your mind which one to get.
Key Differences
Although they look similar and they both have "T" in the name, they are also quite different. If you wonder why the "T" is in the name of both, its simple - they have both titanium housing.

Many other features however differ.
Design
The 508T is open reflex sight, which means for you that it has broader field of view.
The 509T is enclosed reflex sight, and the selling point of it is that it has superior protection against all factors. Simply put, you can treat it much more harshly.
Mounting System, a.k.a. Footprint
The Holosun 508T uses the Trijicon RMR footprint standard, for which you can find mounting solutions for each and every gun - whether equipped with optics ready slide, with only the classic dovetail cut, or even just with picatinny base.

The 509T on the other hand uses unique 509T footprint which you can also find for example on Holosun SCRS. Although this footprint is much less popular, you will still be able to find matching plates for optics ready plates for the most popular models. You can find even a mount with Picatinny base with the 509T footprint.

Weight & Size
The differences in this section come mostly from the design differences. Because the 509T is enclosed, it is also quite larger and as a consequence of that & the titanium housing, its also considerably heavier. The 508T weights 56g (1.98oz) while the heavier 509T weights 64g (2.26oz).
The 508T as a result comes out as the more compact and lighter red dot sight. Just by pure measurements the 508T comes out a bit longer. The 508T has 45mm in length while 509T is 40.9mm long.

Common Features
What do these two have in common? Quite a few things as well:
Titanium housing
Both of these optics use titanium housing, promising extreme durability. Holosun 509T is even enclosed, thus making it double extra durable.
Multi Reticle System

At both of these sights you can switch between 2 MOA dot, 32 MOA circle, and combination of both.
Available with Red Dot and Green Dot
Both of these red dots are available in red and in green red dot variant:
Solar Panel
You will find solar panel for backup on both of these.
Automatic Brightness Adjustment & Motion Sensor
That does not require further comment imo.
Battrery
Both of these have battery housing from side, and both use CR1632 battery with life expectancy up to 50 000 hours.

What to Consider
When choosing between these two, you should consider what you want from the red dot:
- Treatment: For extreme conditions, go for 509T
- Use: For versatile use and wider field of view, go for 508T
- Mounting options: And foremostly, make sure there is an adapter that will make the optic compatible with your model. The 508T is compatible with very widely used RMR footprint, but the 509T mounting solution can be a bit tricky to get. Make sure to double check the availability before you get one.
Video & How to Use
I also made a video which includes basic guide on how to operate both of these optics. You can find it on Optics Spot official Youtube Channel.