Which is harder/more time-consuming in college? GPA killer?
Depends.
If you just want a BS in Statistics and then stop there, I would say that they are both pretty close, with Stats maybe being marginally harder.
If you want to go on to Grad School in Statistics, then you will basically have to double major in Mathematics, or at least take two semesters of Real Analysis, as much Linear Algebra as possible, and maybe even some Computer Science courses (usually not required for a BS in Stats.) Then, by far, Statistics is harder (ie not as many people have the nads to get through.)
Im doing a math degree with the hope of double majoring in math and a minor in legal studies.
Like the previous poster said stats can get pretty full on especially in the grad level. If your interested in stats and economics you might want to consider actuarial studies which is basically a tonne of stats and some business subjects. To some its fairly boring and because of these and other factors though it has a high dropout rate and exam failure rate amongst participants. But they earn a tonne once they are qualified and the people that like the challenges do find that it can be interesting work. I'm studying some of the stats that they study but i find things like accounting and financial stuff boring so I know I wouldn't really last doing it.
Economics math wise can be easy or hard depending on the program. If you end up doing some serious modelling then you need to be fairly proficient in math learning things such as tensor calculus as an example. I don't know about the social side of economics however but if your thinking is that way inclined as well then that's a definite advantage.
Also with statistics you have to be able to interpret what you see. Theres a lot of judgement which isn't always clear cut. Probability as a subject is a fairly difficult one especially when you get into graduate probability (the type that skilled financial engineers use). Even simple probability can be not that intuitive and requires you step back and really put aside your natural intuition for some mathematical analytic substitution in thinking. A dice roll for example is fairly intuitive but a question that may involve say three applications of probability calculations is not always that clear cut so keep that in mind.
Anyway I hope that helps to some degree. Good luck with your decision and your future.