Is the 3/4 inch Mepitac soft silicone tape wide enough for a Tummy tuck scar, or would the 1 1/2 inch tape hold better??

wider one -1.5 inch will be better . Narrow one may slip out of the place . So go with the wider one.

Thank you for your question. Both would work fine, I would choose the one that's cheaper.  Each size roll will provide you with plenty of length.  

Either tape should work well for the tummy tuck scar. the dimensions would be fine over the incision to help with the scar remodeling.