Day/night crossover is in reference to: Do you want to use the same gun in the day and in the night. Not, do you want to use thermal in the day.
The reason being:
With a stand alone sight, you have dedicated that gun to night use, and pretty much have to switch between day and night guns. The benefit is lighter weight, and better center of gravity.
With a clip on, you only need to keep one gun handy, and just add/remove the clip on as needed. Thats WAY handier as a concept, but makes for a more front heavy gun.
Cost. As a general rule, you can get a stand alone thermal cheaper than a clip on thermal in that same quality range.
If you’re really expecting to reach anywhere near 600 yards, you’ll want to allocate more budget, both for thermals and for a night ranging system like a rifle mounted range finder.