They aren't even in the same ballpark.
Viagra makes it easy to get rock hard and remain rock hard for long periods of time. But it does nothing to increase libido itself, and at least for me personally, it makes it impossible to reach orgasm. I could rail my wife for hours on Viagra and never cum. Which might sound fun if you are used to orgasming quickly, but it's actually a nightmare.
Mt2/pt141 actually increases libido. You will be more horny than you've ever been before. As a result it will be easier to get hard and stay hard, but every sensation, whether physically, mentally, emotionally, etc, is much more erotic and pleasurable and turns you on a whole lot more.
The injection that causes automatic permanent erections (priapism) is trimix/Caverject. It causes erections regardless of whether you are turned on, so you could be watching a Chihuahua eat it's own vomit, it wouldnt matter, as long as the drug is active, you will be rock hard. There really isn't a strict need for the hospital if it lasts more than a few hours, as the priapism is high flow so the penile tissues are fully oxygenated, but after about 8 hours it will hurt so bad you will want to go anyways.