Sugar – Correlation Between Perceived Sweetness and Sugar Content

White beet sugar and white cane sugar are virtually identical in composition, but there may be very small differences (~0.05%) which some cooks find affects caramelization. Reportedly, cane sugar will caramelize better than beet sugar in many cases.

The bigger difference is when you look at brown sugars. In beet sugar, molasses is added after refining to make the brown sugar, as the molasses that comes from beet sugar is not fit for human consumption. Cane sugar is simply a less-refined product where the molasses has been left in the product. So when baking with beet brown sugar, often the molasses hasn't fully penetrated the sugar granules and "rubs off".

You've got to understand what the purpose of the ingredients is. In a crumble or streusel topping (my go-to in the category is an apple crisp), the (gluten/wheat) flour, butter, and sugar are actually all binders/structural components. Taste/flavor is part of it, but as in your experiments in the other answer, you really can't vary one element wildly (separate from the others) without significantly changing the results.

With that in mind, the butter in a crumble/streusel topping will affect the tenderness of the crumb (similar to cake/pastry). Tenderness will also be affected by whether you used cold, melted, or softened butter (or a non-butter shortening, like margarine or lard/Crisco). The tenderness goes to mouth-feel, which will affect how/when your tongue picks up on the diff flavors.

Similarly, the temp/state of the butter/shortening affects how & how much the sugar will dissolve/interact with it. Depending on what type of sugar you're using (granulated/white, dark/light brown, raw/demerara, liquid sweeteners like honey, corn syrup, molasses, or agave), cold/melted/soft butter will just have a different holding capacity for sugar, which cooks can take advantage of to "super-sweeten" or "under-sweeten" given fundamentally the same amount of sugar.

And of course in a crumble, your perception of the sweetness of the finished topping will be impacted by the filling, since there's (delicious!) flavor interaction during cooking.

So it comes down to ingredient preparation & choice and cooking techniques - not just raw proportions. Cooking is a sort of applied chemistry - sometimes how you put things together is as/more important than what you put together.