The below is specific to the testing as done by Trainer Road. Different protocols do different warmups and pre-testing clearouts, so if you are doing it on a different platform, you might find it very differet. Also, n=1.
I started using the ramp test last year when it arrived in Beta, and I really liked it. It's much more mindless than the 20 minute test - you just keep to the power until you can't. Also, it's quicker (over in about 25 minutes) and less TSS, so you can either tack on another short workout afterwards, or leave it as a low TSS day.
However, I think it suits people who have less experience or have maybe changed their FTP significantly. You also need to be fuelled and psyched up for the last couple of minutes of suffering - they make a massive difference to the final number. I actually went back to the 20 minute test last time I did one as I've been doing Sustained Power build, and I think I had better endurance near threshold but without the pop at the top end. I also had a couple of ramp tests where I quit before my legs gave up. One significant thing to be aware of with the 20 minute test in TR is that if you stick to the power it sets for you, you get an FTP decrease! You need to set the intensity to 105% or so in order to get the same number as you started with!