Assuming every application of either is the same, yes.
However, many of those pre-mixed fillers have talc in them. In polyester, used where it is exposed to moisture, talc isn't a good choice as the 'filler' in any filler product, pre-mix or not.
BigMac will be doing quite a bit of poly and glass work on his Capri, so why buy a product if you've got most of the necessary materials on hand to make it yourself?
Pre-mix is expensive by volume compared to you mixed
If you buy pre-mix you've got a fixed volume of filler. What if you buy too much or too little?
Pre-mix is a fixed consistency so you may need more then 1 type. Fine for fairing, medium for minor filling, and heavy (hairy) for structural filling along the edge of the deck, along stringers, between the hull sides/bottom and the transom, and making coved fillets at other 90 degree transitions.