Why do you need to pick between a struesel topping and marshmallows? Why not both?
Wash the sweet potatoes and cut into medium-size chunks. Boil these about 15-20 minutes until the potatoes are fork tender. Spread them out on a tray to cool slightly before peeling.
To a large mixing bowl, add the sweet potatoes, margarine, sugar and vanilla and mix well with a mixer. Make sure the mixture has cooled some before adding the eggs and mixing well. OPTIONAL: Add the pineapple tidbits and mix. Spread the mixture into a sprayed 9 x 13 baking dish.
To a small mixing bowl, melt the butter in the microwave. Add the light brown sugar, flour and chopped pecans and mix well until the mixture is crumbly. Sprinkle the mixture over the sweet potatoes.
Bake in a preheated 400 degree oven for 15 minutes. Remove from the oven and sprinkle the marshmallows evenly over the dish and return to the oven for another 15 minutes.