I am not a lawyer, and you should consult a lawyer before you start naming your competitor's brands and products in any publication. It probably isn't worth it; especially if they are bigger than you and it doesn't cost them much (relative to their income) to be trigger-happy with the lawsuits.
I'd also be careful what you SAY about the other product, any denigration of their product or any claim of superiority, no matter how small, can lead to them filing a lawsuit against you. Whether what you wrote is true or not. And the discovery phase of the lawsuit alone, before you ever get to talk to a judge or anyone else, can cost in the tens of thousands.
Heck, Papa John's Pizza was sued to hell and back for years, spending millions on lawyers defending their slogan "Better Ingredients. Better Pizza." They prevailed, but as I recall they did not recover their defense expenses.
All of those warnings said: You should put the trademark symbol on it (r in a little circle, if that is how Levi's presents it (probably so, a registered trademark, but go check their ads or publications) or the little "tm" if that is how they do it. Also change the text "Levi's 501" to emphasis (italics probably) to delineate what is trademarked. I'd do this every time it is referenced. On the first reference, you can add a footnote (7 point font or greater, in the USA, I believe that was the law decades ago when I was writing advertising copy) to note "XYZ is a trademark of ABC corporation."