I've owned both (at the same time). The HD25-SP is relatively neutral, with darkish highs. The HD25-1 has boosted (but not overbearing) bass, and clearer, more extended highs.
Nearly everybody will like the HD25-1 better, most of the time. I find the sound very "involving" on pop, rock, jazz and the like. It's really enjoyable. However I think the HD25-SP is the better can for classical, for solo piano, or for any other genre where a boosted midbass throws the balance of the music out of whack.
The HD25-1 is easier to drive. The HD25-SP benefits more from an amp, which helps lighten its darkish presentation.
I frankly think it's a pity that the two have similar names, because it inevitably leads to them being compared, with the HD25-SP coming out behind. Yet taken on its own, the HD25-SP is a good little <$100 closed headphone; before the advent of the K81DJ I would have suggested it to anybody as the best <$100 closed portable out there. In fact I like its sound better than that of the HD280 except when the latter is driven by a good home amp.
Other details: they offer pretty much the same level of noise isolation. The HD25-1 is more comfortable, having far better headband padding plus that spiffy adjustable split headband. It also will do a better job of staying on your head if you use it while exercising or something. The HD25-SP packs down smaller and is less conspicuous on the head. The HD25-SP comes with a 3-meter two-sided cord with straight plug, and the HD25-1 comes with a 1.5-meter one-sided (right-side, which is unusual) cord with a right-angle plug.
Of the two, the HD25-1 is much more like a closed version of your PX100. The HD25-SP is, well, a little like a better PX200: good-sounding, but dull compared to the HD25-1 or PX100.