Honestly, I have only ever used the illumination function on a scope in a practical manner once. And that was at the Guardian long range night match at Frontline Defense several years ago. I guess if you are going to hunt/shoot at night periodically, then illumination is a good feature to have. But for most folks, like myself, it will probably never have a recurring practical use.
As for optical quality, the Diamondback and Helos seem to be about the same brightness and clarity to me. Also, the clicks on both are about the same - neither is mushy. I intend to mount the Diamondback onto the 455 this weekend and test its tracking precision and then run it out to 500 and back. That should tell me a lot about its quality. So far, I have only played with them right out of the box, and have not actually messed with one that was mounted to a rifle.
As for the Argos vs. the Helos and/or the Diamondback, the Argos has the advantage of a zero stop (and also illumination vs. the Diamondback, if you are into such things), but sometimes at the expense of much mushier turret clicks. Ultimately, you'll need to decide what scope features you cannot live without in the application you are gonna be using, then start the process of elimination from there. Make sure you buy what YOU need, not what everybody else wants.
For me, I think the Diamondback is gonna end up being a better fit for my rimfire trainer and I will move the Helos to either a 6.5 Grendel or heavy SS .223 Wylde AR upper for additional mid/long range gas gun options. Your mileage may vary.