Audio Research LS15 vs. SP14

I’m not sure about the production date for these preamps but they seem to be within the same price range in the used market, basically within a $1200-$1500. All things being equal, which one do you folks prefer in terms of sound quality. I’ll be using it just as a basic preamp with an SS amp, ML Aerius speakers and a CD player so I don’t need a lot of features, just sound quality. Thanks.

Edited to add Audible Illusions Modulus3 as another option although it's priced somewhat lower than the ARC components referenced.

The Audio Research has most of the information you need. The SP probably has a phono stage the LS does not have a phono stage
there were also two versions of the LS15, most easy to distinguish by the removable power cord on the later version.  there was also a cap and power supply upgrade
Yes, a nice tubed phono stage in the SP-14 . Also, given the aluminum chassis, it’s very easy to install an IEC socket(ie: C-14), to facilitate the use of an upgraded power cord. 
A point to be aware of is that as with many ARC preamps and line stages, the LS15 and SP14 have output impedance characteristics that may make them unsuitable for use with some solid state power amps having lowish input impedances.  Specifically, according to the information at

LS15:  "40K ohms minimum load and 1000pF maximum capacitance"

SP14:   "Recommended load 60K - 100K ohms and 100pF. (20K ohms minimum and 1000pF maximum)"

The load capacitance recommendation (as opposed to the load resistance recommendation) probably won't be an issue unless the length of the cable to the power amp is particularly long.

-- Al
P.S: Stereophile’s measurements of the output impedance of the Audible Illusions Modulus 3A was around 1800 ohms, presumably at a mid-range frequency and therefore probably higher at some other frequencies. If the Modulus 3 was similar in that respect it would also raise concern about impedance compatibility with some and probably many solid state power amps.

-- Al

I had the SP-14 in the system for a number of years before moving up to the REF-3.   The SP-14 is a fine preamp with an excellent phono stage.  I ran it with a number of amps, including an ARC Classic 60, Atmosphere OTL 60 watt mono blocks and even two of Frank Van Alstine's biggest stereo amps bridged.  For the money they bring these days on the used market, I'd consider the SP-14 to be a good choice, especially considering the excellent phono stage that comes with it.
I had an LS-15 for years, then upgraded to an LS-25.  The -15 was a good amp for the price, if not a little sterile and on the cooler side.  It did eat tubes, though, as did my PH-3 Plus.  Annual replacements were a must.
Someone mentioned having an LS15/PH3 combo and that they burned thru tubes at a rapid clip.  I had the same combo for years and never experienced this.
I just bought one LS15 from Audiogon. Interestingly, I am also considering add a PH3 for my phono stage.

Not sure why preamp burns tube like French fries, I just can’t believe it.  These are small tubes and not for power amplification. 
I have a LS15 then moved up to a LS25mk1 never had a problem with it eating thru tubes. I have used JJ Tesla, Ei Elite and some Amperex 7dj8 tubes with no problems ever.