I have been offered two PhD positions with two different subjects, which one is more useful, applicable and has a better future both in academic and industry?

1- "FEM on Surfaces:surface diffusion, surface tension, elastic shell"
the aim is to develop and study new, structurally simple FEM on surfaces, which will focus on computing the actual surface instead of mapping between the surface and the plane

2-"Multi-objective optimization of machine components" and "Robust optimization"
Category
Education & Reference > Higher Education (University +)

Additional Details
The Fund is exactly the same! Both are in the same university.

Congratulations on being offered two positions, that is extremely fortunate! My only advice would be, job prospects aside, which do you think you are more interested in and can live with for 3 years? You need to study something that will keep your interest, and keep you motivated. It's a hard old slog, especially when you get to your last year, so make sure it's something you really want to study! It's all very well having better academic/industrial prospects but you have to get to the end first!

lindalou is right. Another thing you might like to do is check the job market and see which of these projects is likely to give you skills that are in demand. Also, develop an awareness of where things are going in your field, as you need to be readying yourself for applying for the jobs that will exist in 3-4 years time.

If I were you I would try and visit the University, meet with the people in the groups and see which group I would see myself working in. This is a major factor in a PhD