in terms of nail health neither acrylic or gel should harm the nail if the proper products are applied and removed correctly, in terms of popularity in my area I would say its gel polish,then acrylic then gel overlays, no one offers gel extensions locally to me but popularity will vary by area
profit is also a bit tricky because different brands have different costs per service and will take different amounts of time depending on what you are good at. For example on my level 3 course the other two girls doing exams the same day as me preferred acrylics and can do them quicker than gels but I was/am the opposite so gels work out better for me from an income point of view but acrylics work out better for them
if your main concern is income generation you may be better off starting with gel polish and seeing what your client base prefers or perhaps see about doing level2/3 course which covers both so you can get some hands on experience (I got a 24+student loan, if you go for something similar you may not hav to pay back right away)