I owned a 2012 Sierra 6.2L, 6 speed, a 2017 ram 5.7L 6 speed, then an 2018 Sierra 5.3L 6 speed, and now a 19 5.3L 8 speed Sierra, and the 8 speed 5.3L feels almost as fast as my 2012 6.2L did. I don't regret buying the 5.3L this time around. I still love it just as much as I love my 2012 6.2L.
I raced my friends 2012 5.7 Ram (6 speed 3.92 gears) a month or so ago, in my new 19 5.3L, and we were dead even and hes got a custom tune, long tubes and CAI, but he is running a 3" lift and his truck weighs about 8-900lbs more, but has a lower payload and the same tow capacity. Considering all the other mods, hes pushing out at the very least 400-410hp, and my 19 5.3L 8 speed, 3.42 gears, was dead even with him.
My 2018 5.3L 6 speed felt a bit slow though, it felt a lot weaker compared to the 2019 8 speed and for my 2018 I did wish I got a 6.2L, but when 19 came around with the 8 speed I figured I'd give the 5.3L another chance since the price was right. GM really improved the responsiveness of the 5.3L in 2019 by matching it with the 8 speed.
ALSO!! Great news!! It looks like the new Scorcher tuner for the NEW 5.3L L84 motor with DFM, is being released at the end of the month. I noticed on their site is now says shipping begins on October 31st 2019, so if that's correct, the possibility of tuning and real aftermarket mods just opened up.
Also, if you think the new 5.3L might be underpowered, which it isn't, but if you're worried and dont want to fork out the extra for a 6.2L, get a flex fuel 5.3L, I think its like a $100 option. Running E85 in the 5.3L it makes around 380hp and 415ft lbs of torque. That's pretty damn good power and more than most people actually need. The 6.2L is a great motor, but so is the 5.3L with the 8 speed, and it you run it on E85, it feels pretty damn powerful.
If you've got the money, go got the 6.2L, but these new 2019 5.3Ls are a different beast compared to the 18s and I don't regret giving the 5.3 another chance. It's got enough power to at least smoke the young kids in their old civics, I even beat my buddies 2004 Infiniti G35.