I have an SL-1000, SL-1025 and an SL-1015. All are complete with the factory plinths and arms with the exception of the 25 which had the EPA-100 arm removed in favor of an EPA-500 which has the "S" shaped wand with the detachable headshell. I listen to the 25 the most, almost daily. The SL-1000 is a little tighter on the bass and does indeed have more torque than either of it's baby brothers. I prefer the EPA-100 arm that's on the SL-1000 and account a lot of my impression of the table to the arm. I can't imagine somebody saying the EPA-500 is a better arm than the EPA-100 but we all have different ears and mine obviously prefer the 100. As for the SP15 (SL-1015 in my case) I don't think the prices really justify what you get. People like them because they have a digital pitch adjustment but the SP25 has a manual adjustment which works just fine. The only real benefit to the 15 would be if you were transcribing 78's, which I don't do. My 1015 has the EPA-500 arm with the "H" arm tube. I've been messing around with different M/M carts trying to match the compliance to the arm. When I swap the "S" shaped arm from the 1025 onto it, it sounds remarkably like the 1025. Internally the 25 and 15 have just about nothing in common. However, sonically they are very similiar. The 15 is a little better (tighter bass again) but you'd have to twist my arm to get me to say it and I'm not sure I could pick it out in a blind test.
On a side note, the SP15/SL-1015 have an IC which when it goes bad will make the speed erratic. The part is NLA from Panasonic. I've had two tables go crazy on me. The SP25/SL-1025 has the motor from the venerable SL-1200 and I've never seen one go bad.
So, there you have it. If you can afford it and want a very nice deck I'd buy the SP10/SL-1000. If you don't play 78's I'd say go for the SP25/SL-1025. In the SL-1025 version it really is a nice deck. The SP15/SL-1015 is just too problematic for me to recommend. Prices on any of these tables have been climbing in the last few years. The best values currently lie in the SL-XXXX versions which have a plinth and (EPA) arm. Often these factory versions sell for as much as the arm would fetch.
On another side not, the SP15 and SP25 can often be found with a "Pro Base" that I think (I'm not sure) was marketed by Audio Technica. It's just black painted MDF. They usually have an Audio Technica ATP-12T arm with this combo. Not a bad set up. Not as nice as the factory plinth/arm combos but a good introduction and value for the money and easier to find as radio stations used them.
Well that's my take on it. I hope I didn't offend anybody. I really like these decks, obviously, and have put a lot of time into them. When people come over and see all four of my 'tables sitting there (there's an SL-110 with an SME arm as well) I always say my choice in 'tables was akin to not knowing which model of Mercedes to buy...and then just buying one of each.