The effect of cooking and soaking on vitamin/mineral content of foods
In general, boiling decreases the amount of vitamins and minerals much more than soaking.
Boiling can reduce the vitamin content of a food by 25-70%, but this can greatly depend on a food (NutritionData).
I haven't found data for oil seeds, but according to USDA Table of Nutrient Retention Factors, boiling of rice can result in 25% loss of vitamin C and B1 and 40% loss of folate, and boiling of nuts in much smaller loses.
In one study about potatoes:
Leaching alone did not significantly reduce levels of potassium or
other minerals in tubers. Boiling tuber cubes and shredded tubers
decreased potassium levels by 50% and 75%, respectively.
In seeds, the results could be different, but soaking would likely have a small effect.
In conclusion, I assume that soaking seeds would not result in any significant loss of vitamins and minerals and boiling them for 20 minutes in a bit greater but still relatively small loss.
The effect of cooking and soaking on the level of antinutrients in foods
According to this study: Effect of soaking and heat processing on the levels of antinutrients and digestible proteins in seeds of Vigna aconitifolia and Vigna sinensis (Food Chemistry, 1998), soaking for several hours and cooking for 30 minutes can be similarly effective in removing antinutrients.
In V. aconitifolia [moth bean] soaking in distilled water for 6 h and
cooking for 30 min reduced the phytic acid content by up to 43%.
Maximum reduction in the level of phytic acid (36%) was observed under
distilled water soaking compared to cooking and autoclaving in V.