I've owned both. Both are great. 2018 3.6 my current 2021 is 2.0 turbo.
The 3.6 is certainly smoother, with the turbo you do get the typical turbo lag so if you punch it from a stop it does tend to "kick in". So a bit less smooth from a stop if you want to go fast. If you start at a regular speed then you don't feel any turbo lag. Around town I find the 2.0 actually more peppy. I like the feeling of it in city driving. I really don't mind it. Fuel economy wise I can get 800km/497 miles on the highway out of one tank. Yes it does run better on premium fuel as suggested by GM. But I use premium on all my cars anyway so that doesn't bother me except when it gets to $2/litre. Then that becomes expensive, like $120 to fill.
So typical daily driving they are both fine and I liked them both with the turbo actually a bit more fun in the city.
On the highway is where you'll notice the biggest difference. On the highway at any speed the 3.6 has plenty of power left behind to pass in overdrive. It's great. The 2.0 doesn't quite have that same passing power and you are best to have it in sport mode when highway driving to give a bit more omph when passing. Now don't get me wrong, it's fine, and I've done a lot of highway passing with the 2.0 and never thought oh no... I'm out of juice. But it certainly isn't the same level of confidence than the 3.6. Now I'm talking about passing on a two lane highway where you NEED to pass and get out of oncoming traffic quick. If you're on a four lane or more then passing with the 2.0 is not a problem. It's perfectly fine in those conditions. On a two lane you need to punch it, pass then get back in the right hand lane.
If there were two XT5's with the exact options I wanted and one was 3.6 and one 2.0, I'd probably go with the 3.6. That being said, I bought my 2021 in Nov 2021 right when the dealer lots were starting to become really empty but pricing wasn't as bad as it is now and in 2022. So I didn't have a choice, the XT5 had the exact options I wanted and was the only one in 500 km around me that I liked. I was lucky and got 0% financing on it as well. I have zero regrets, love the XT5 and am perfectly fine with the 2.0.
So all things equal, I'd go with 3.6. But if you find the options you want and the price is right on a 2.0, don't hesitate, it's a great vehicle and I certainly wouldn't call it underpowered.