I've been hankering for a taste comparison ever since I fell in love w/ Doughnut Dolly's Naughty doughnut, which reminded me so much of St. John's (London) legendary custard doughnuts, which I (and I assume I'm in good company here) had long viewed as the best in (international) show.
I talked to Doughnut Dolly's owner/creator, Hannah, about St. John's, after my first bite of a DD Naughty doughnut--and she knew about them and about the recipe in St. John's second cookery book--she had never had one of their doughnuts, but what she had heard/read was a guide, as she developed her own recipe.
So here I sit in London, still licking the sugar from St. John's custard doughnut off my fingers, so as to spare my keyboard.
Both St. John's and Doughnut Dolly's are terrific (brilliant, in British vernacular). Similar creme fraiche flavor to the filling. Similar pastry style.
DD's are smaller. They have a tighter, drier crumb and firmer custard. St. John's are already filled, while Dolly, of course, fills to order, which accounts for the wetter inside of the St. John pastry. Apart from that, the St. John's pastry is floppier and heavier--which is to say, DD's are lighter, w/ a less greasy mouth feel. St. John's filling oozes in a luxurious way, so you're doing more "clean up" with tongue and fingers, which go directly to mouth, a messy and delicious "problem."
My verdict: Dolly's are better--and the best!