tl;dr This is a black box financial model driven by naive assumptions. Anyone can teach themselves how to recreate these calculations in Excel. The real value comes from helping people make better assumptions to drive the model instead of performing the calcs. Otherwise, it's just "garbage in, garbage out."
These calculations are a pretty black and white implementation of finance concepts taught in an introductory course. The most complicated math involves compounded growth and amortization. Granted, the data entry and presentation are well-laid out, but I could make a similarly easy to use Excel spreadsheet in 30 minutes with the ability to control all the model's drivers.
So I agree with others who have pointed out that you have made a very usable, albeit overly simple, financial calculator, but that's a fairly low barrier to entry product given the ease of the math and concepts.
As a real estate finance guy, I get asked for advice on home-buying all the time. The calculations are easy. The real value in my advice, or any financial advice for that matter, is help thinking through the assumptions that are used to power a financial model, not actually doing the math itself.
The drivers of your model are:
1) assumed comparable rent
2) home value appreciation
3) rental appreciation
4) financing (size, rate, amortization, and term)
5) annual expenses (I assume you inflate these at 2% too, but it's not listed)
6) and cost of capital (what you call "return on savings")
Right now, a majority of those variables are treated as an afterthought when they should be the main event. Therefore, I find the results of this calculator to be somewhat dubious. Said differently, I don't think this is going to help laypeople make better, more informed investment decisions because all you've got is a good-looking "garbage in, garbage out" financial model.
My suggestion is to guide people to make more informed assumptions that feed into your mechanical valuation tool. That would create real value. For example, challenge them to consider the determinants of appreciation. Will demand for the region in question outpace supply over the next 10 years? If so, appreciation will probably exceed inflation. Ask questions like these and based on the responses, convert them to quantitative inputs. [Admittedly, my question was probably too technical.]
Separately, your decision metric lacks objectivity. You present the "breakeven" point: the number of years that it would take for owning a home to be better than renting in gross dollars. Why not just ask upfront, "How many years do you expect to live in the same home?" and then give a definitive answer (e.g., "you should rent"). Even better, you could ask a number of questions to determine expected hold period. Age. "How long have you and your spouse lived in the region?" "How long have you and your spouse had the same job?" "Do you have family in the region?" "Is job mobility important to you?" Then convert these to a range of expected hold periods.
As you know, home buying is different from a typical investment in that it is a consumption good as much as it is a financial vehicle. There's plenty of emotion to contradict logic, if not more. If I find a house I have to have, I'll make this model give me the conclusion I want it to give. That's what I mean by lacking objectivity. Worst case, I'll convince myself I'll stay in the region for 5, 12, 17 years, whatever your mechanical tool tells me.
My heuristic right now to friends and family: if you don't need mobility (job or otherwise), buy and borrow as much long-term debt as possible at these incredibly low rates. If inflation and interest rates spike, you'll make out really well. If inflation and interest rates stay where they are, you won't be much worse off. If you need job mobility, rent.