I am an American citizen expecting to take my Canadian citizenship oath hopefully soon. I am doing it because I pay taxes in Canada and I think I deserve the right to vote.
But more than that, I am also doing it because you have to renew your PR every (I think) 5 years. While it is not overly expensive, it is a process. Citizenship is a process as well but it's a one and done process. I would say (not taking the waiting and the fees into account) the hassle for the two are about the same. So, I can go through the hassle of renewing my PR every 5 years (and run the risk of losing it by unwittingly not meeting residency requirements) or I can go through roughly the same amount of hassle and be done with it forever.
Also, a small note on renouncing your American citizenship once you become a Canadian citizen. I plan on keeping mine. I can see the benefit of renouncement. It is a bit of a hassle to essentially have to fill out your taxes twice. Which, BTW, you have to do as a PR as well, so remember!
However, the IRS form for Americans living abroad is fairly simple and you don't actually end up paying any American taxes on foreign earned money until you make something over 100k american, or like 130-150k Canadian. And even then, you get an additional tax break if you own a home. And THEN they start taxing you on your income OVER that amount.
Now, if I were to end up making 200k a year American, I might THEN consider renouncing my citizenship. I do not foresee that happening any time soon. And if or until you cross that threshold, unless you have some philosophical reasons, you may as well keep both.