A 0.5% difference is quite a lot. Actually, the differences between the offers are not that big. They are usually more in the 0.1 range and depend on what you can do with the money. There are providers who don't really care what you buy with it ([Kitchen, fireplace, etc. all no problem]), who also don't want every euro accounted for, and there are others where you need a special permit if you deviate from their payment plan (which NO builder accepts).
Currently, interest rates are historically low, but they have been rising significantly again in the last few weeks (0.2-0.3% for 20 years). Yes, a 0.5% difference means 125€ less interest per month; over 10 years and 1%, it even amounts to a whopping 250€ per month. But what happens after the 10 years? In that time, you have saved around 25,000€ in interest. Yes, a really great sum that could be put toward repayment. But what if the rates are 3-4% then? Or even higher? What then? You must not forget that the 10 years start running as soon as you sign the loan application. If you build for one year, you only have 9 years of repayment. That doesn't add up to much.
Instead of focusing on the last decimal point of percentage, it is better to ensure that the overall package fits and that you remain planning-safe. If the amount is low and/or the rate is high, you can of course also choose a very short term (and low interest) and take a bit of a gamble. For a typical 25-year financer for 300,000€, a 10-year fixed interest period today would be harakiri (unless you expect a really big influx of money - e.g., inheritance).