>>64114099
The side rail is my personal preference in general, but either option will be good as long as you buy one of the good dust covers (TWS, Zenit/Barwarus, I've heard good about Midwest but I have no experience with it) and it has a tight fit to your rifle.
RS Regulate has availability issues a lot because of high demand and low production (it's just one guy afaik) but is g2g in terms of quality and function. If you find what you need in stock, buy it; Primary Arms had good prices and availability when I needed a new upper half, shop around.
There's multiple upper halves, some are purpose-made for specific optics, some are just a picatinny rail. If you're after "simple" like a red dot, the RS Regulate is going to work fine and might work better depending on what optic you choose. Anything without a specific upper mount (like an LPVO) does sit a bit high, so ymmv.
It's mostly down to aesthetics anymore, if you're afraid of losing zero just don't take your optic off.