This question boils down to the fundamentals of property investment and investment decision making in general. Here is my 2cents:
It depends on how the property fits into your portfolio. If that 100k represents 50% of your net worth, then i suggest selling it (too risky to keep). If that 100K represents 10% of your net worth, then its not so cut and dried. This is a risk management consideration. Is this property the only rental in your portfolio? If so, then you are divesting fully from property. what will you do with the 100k if you sell? (what is your opportunity cost and how much will that investment earn?). Do you have a favourable mortgage with this property, and if you decide to buy later will you be able to obtain and equally favourable rate?
I do not understand your calculation for 15-20k extra after 10 -12 years. If the property is worth 260k now and you owe 150k, in 10 years we can assume if the property increases in value at 3% per year (historical average) it will be worth roughly 350k, and if the renters are paying down 5k/year then you will owe roughly 100k. That means in 10 years you would receive 250k in non inflation adjusted dollars (worth 185k today after adjusting for assumed 3% inflation). Then you need to consider the opportunity cost of what you will do with that 100k over the next 10 years. If we benchmark it against a safe 3% bond, then that 100k will total 134k after ten years, which basically means it has the same purchasing power in 10 years as today after adjusting for inflation. In order to equalize the investments, you would need to be earning a CAGR of 11% with that 100k over 10 years to match the property investment without accounting for the difference of the capital gains tax. Even so, thats a daunting number to hit if you sell without the benefit of the leverage provided by the mortgage.
Will property continue to rise at the historical average, or grow slower or faster?
Will you earn at least 11% with that 100k for ten years?
How does the property fit into your overall portfolio?
Can you manage risk effectively, or need to sacrifice diversification in order to achieve that yield?
These are the basic questions you need to consider before making any investment decision really.
Good luck with whatever you decide.