According to the Guardian articles I referenced earlier, treacle sponge is a classic English steamed pudding while STP is originally from Leftpondia and is more like what they call a muffin in texture.
Steamed puddings can be a bit heavy in my experience, but that's probably down to how they're cooked. I'm vaguely aware of the canned versions but not sure I've ever had one. The STP I made the other day was surprisingly light.
I think both have their place.
We always used to have a can of sponge pudding in the cupboard when I was growing up. It was only ever consumed on very special occasions and it was partitioned the usual way: my dad got half, I got a quarter and my sister got a quarter. My mother has rarely ever eaten. This was always the grand disappointment of that other canned glory of my childhood, the Fray Bentos pie. The paltry quarter, if that, my dad's share took half the remaining filling too. It was, at least, better than my mother's meat and potato pie in which the meat was basically undissolved lumps of bisto.
Anyway, I mostly ate pudding at school. I can't remember the details, I think there was a lot of Manchester tart (if I recall, pasty smeared with raspberry jam under an adipose layer of cold congealed custard and sprinkled with that recognized signature of Manchester, a dandruff of desiccated coconut. Possibly random fruit crumbles in which the race was to get crumbly crumble – not the second sitting smush.
Bananas in custard. I think bananas in custard was main dessert I ever had at home. Until the time my mum was chased by a tarantula, after that bananas were off the menu, and we resorted to Angel Delight, a dessert option never knowingly threatened by the appearance of large spiders.
I don't especially like sweet things, they make me a bit sick and cranky. Even when I have dessert, a few spoons in, I'm mostly starting to regret it. I once ate a giant ice cream in Chicago, and it came back up fifteen minutes later as I walked down the street. Trying to stop a very public barf, I clamped my mouth shut so it took the path of least resistance and came out of my nose. I was like a human ice cream truck. Could have filled two cornets simultaneously. If I could have rotated my head three-hundred and sixty degrees, I could have been a Dairy Queen Linda Blair.