What do you already offer? I personally found lashes cheaper and less time consuming to get up and running but there's a lot of demand for nails where I live. If you already have a bed you take around then I would recommend a lashes course, I find it ties in nicely with brow tint and facial waxing for me and I only have my bed and stool for set up. If you already have table and chairs then look into nails - personally I think you have to put more work in and build skills over time, but a lot more people seem to want nails - maybe because they have experience with getting them so are comfortable, I'm not sure!
If you opt for lashes I would recommend also doing lash tint (if you don't already) as it looks much better if you have clients with super blonde lashes if you tint first. It's also a quick and easy treatment to offer as a stand alone when doing other treatments