Ask HN: Should I incorporate or operate as a sole trader?

I have a full time day job as a industrial/product designer that I love and don't forsee myself leaving anytime soon but also love to develop web software in my spare time. About a year ago I came up with a SAAS app idea that we would find useful at work; we spoke about the idea and everyone likes it, but web software is very much not their business. I recently started hacking it together in my spare time and am almost ready to get our office using it. I then intend to polish it a little more and try to get some paying (£10ish a month for a business with 10 users) customers. My inital plan is to get to the stage where it will pay for my daily 2 hrs train commute where I have done, and probably will do, most of the work for it, although I think it has much more potential than that. My question is should I incorporate as a uk Ltd company or operate as a sole trader? I don't think it will affect getting a merchant account and setting up the subscription payments but the initial setup costs would be higher. What have other people experienced?