Buy an Audio Research SP10 MkII or an SP11 MkII ?

put the tubes on the floor and roll... which tube goes further wins : )

What sirspeedy means is swapping different brand and model of tubes (like tires on your car) and you'll get different performance. Some do better in (wet, ice, dry pavement, sand..) clarity, body, extension, and presence.
It takes a lot of lab test ( trial and error ) to get the best performace you want for your taste of music.
It took me over 50 different 6DJ8/6922/7308 (except CCa because they're too expensive) tube families to get the way I like on the SP-11 MK II. I'm not big fan of SP-10 because it sounded a little slow and thick for my taste.

I'm the guy Jeffrey Behr referred to above and while it's been almost 20 years since I last heard an SP-10 (and 14 years since I heard the SP-11 Mk II I stashed in boxes back in 1991), having read this thread, I'll offer one more suggestion: If your amp is sensitive enough and your phono cartridge has enough output and you don't load it down too far, you just may be able to get away with using only the phono stage of the SP-11.

I was able to do this with HP's reference system back when I worked at TAS as well as my own system (just remember to invert phase at the speakers to make it an equal comparison) and if my memory serves me correctly, the stripped-down, "hot rod" version of the SP-11 was clearly preferred to the SP-10 that sat alongside it by pretty much everyone who heard the comparison.

YMMV, of course, but if you find a good deal on an SP-11 -- and come to think of it, mine might well be for sale -- it's an alternative you may wish to consider.

Have run my D115 MkIIs with both the SP10 MkII and Sp11 (with a Siemens tube upgrade on the phono side and telefunken on the line). The tube upgrade on the 11 was a noticable improvement. The line stage of the 11 is much better than the 10 IMHO. The phono side of the 11 was also more detailed; I noticed this difference between the 11 and 10 right away.

But if you play LPs, the 10 is the pre for you. Yes, the tubes are expensive (I've gone the amperex route with my 10, but have not noticed nearly the improvement that the siemens upgrade brought to the 11), but the difference in the music between the 10 and 11 was immediately heard. The 10 just sounds better, to me -- way better. I still have the 11 and use it for CDs upstairs, but in my listening room (filled with LPs) is the 10.

BTW you may have already made your decision, if you have, how did you go?