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.

CAT Ultimate or SL1 Sig. Add to the comments above, the CAT offers very good dynamics. Also drives difficult loads beautifully. True, it's more on the "neutral" side than the ARC.