CAT SL MKIII vs ARC SP 11 or 10

Go CAT. I've never heard an ARC come close to sounding as good.

I had an SP 11mk2 and a VT100mk2. I replaced the SP11 with a CAT Ultimate. The CAT has a few quirks, the power supply hard wired to the control unit and a volume control lacking in flexibility, but it's sound was a step up. The bass is fuller and tighter. The soundstage is bigger and more percise. The SP 11 is a fine unit and I don't mean to suggest it isn't a good choice. However, it is more colored than the CAT Ultimate. In some systems the SP 11 may be a better fit, but the CAT is more tranparent to the source.

Well, I have a CAT SL1 Sig Mk3 & I really love it. The phono stage is really quiet, dynamic & produces some really good music.
My friend has a ARC SP-6A. Once Richard Gray (of RGPC) brought it up to spec & K & K Audio changed the coupling caps to Kimber + changed the power supply caps to Panasonics + changed the RCA output jacks to Vampire + added an IEC, the SP-6A sounded very good in my system as well. The bass of the SP-6A is not as deep & controlled as the CAT, the CAT has better high freq extension. However the SP-6A has that ARC signature midrange that has bloom that I have not heard in any other product.
I like the CAT better while my friend likes the SP-6A better!
IMHO, you cannot go wrong with any of your listed choices.
I have read many times that maintenance of tubes in the SP-10 & SP-11 is expensive. Each time the owners tried to save money buying less expensive tubes they felt that they had wasted their money (sound degradation) & that they should have paid the extra money for those expensive tubes. No personal experience - just telling you what I have read.
Search the archives of Audiogon & AA. FWIW.