I think this will be a mixed bag with regards to answers.
For those who have to watch the figures and need the safety of knowing what is going and out and what needs to come in to cover those costs, I think they'll go for a fixed rate.
A fixed rate would also be good for those who may only be interested in capital growth so they could opt for an easy life on a longer termed fixed rate of 3/5 years and remortgage when necessary to ensure they get the best rates IF a rate rise was to take place.
As for variable, I have my 3 BTL's on a variable rate, purely because I've remortgaged over the last few years when the interest rate was either falling or since its been at 0.5% and therefore I've also been in the position to know that at that point, I was going to get a better rate using variable than a fixed.
The next year will be an interesting time and worth asking a similar question in a year or 18 months after the election and see what the view is then. It's only a matter of time until interest rates increase and its at that point when thins could sway more towards a fixed or variable.
For me though, at this moment in time, I'd use either depending on the property I'm looking to mortgage.