I use a mk 18/G250 HP as my primary. I think that most will agree that the MK25 outperforms the MK 18 as far as breathing, but there are other things to consider. Lots of info on the 250 HP versus the 600 on the boards.
I am inland, and have a tendency to dive year-round, mostly blackwater currently. I like the diaphram for this (neither the 18 or 25 are environmentally sealed), as well as the 18's resistance to the cold. Basically, depends on what you are looking for relative to the conditons in which you dive... ie cold, deep, sediment & silt, et cet. Some people have posted that they have had problems with the 25 freezing, ask me about the 18 in January.
I can't speak to the oceanic line. If you are looking for data to compare, Scubapro has/ had the spec sheets for all the first and second stages on the UK web page. I tried to upload the images to the forum, got the size of file error message... send an email if you want them. I think oceanic has similar data in the back of the printed catalogue.
ended up getting the mk 18/G250 HP combo a few weeks ago, about 10-15 dives on this setup. Someone from the board steered me to an authorized SP dealer via my LDS, total price was $290 not including shipping. Absolutely no regrets.
This combo was being phased out in the US, and I think they are phasing it out of the UK as well. I heard rumors of a Mk 17 being developed, but no info on it.