anybody have an opinion? 18" wheels vs. 20"

I don't think there is any real difference. The 20" with shorter sidewalls and the diameter is not much different than the 18".
And with the larger wheels showing up on most trucks and SUVs now, the 18" and smaller wheels are starting to look out of place.

It seems there are more selection of E-rated tires in 18" but it's really the overall diameter that matters. I've had both and haven't really noticed any difference. I do think the 20" on my current track look a bit better and are more to scale.

I did screw up on my last truck when I got 305 width tires, although they were E-rated the max psi was only 65 instead of 80.

I liked the look of the 20" wheels on my old truck, but I don't think they had any impact on towing. I haven't missed them on my new one which has 18". Tires will be a bit cheaper with 18" wheels.