I have always done the 20 minute test and therefore prefer to remain with it for consistency.
As a TTer I also feel it is more representative of the races I will be doing. Results have validated the FTP test results.
I have only done one half-hearted ramp test and I failed at about 220 watts when my FTP is between 240 and 250. I keep meaning to do a full effort at one but long term I will stick with the 20 as that is a proper workout in itself.