The audio quality of an interface can be judged by the combination of its preamps and converters. Manufacturers outsource their AD/DA converters from a hand full of companies. Those converters may share similar components, but that does not make them all equal. Manufacturers also offer different preamp topologies that are unique to their brand and product line(s)/generations.
Specs for the preamps such as dynamic range, THD, cross-talk, etc. are generally published by the manufacturer, which can help differentiate them. Converters not so much, as that area tends to be more of a black box. Of course the sample rate and bit depth options will be indicated, and therefor able to be compared, but some specs like clock jitter may not always be available.
The specs on the Solo do differ from the flagship x16, as well as other entries in the Apollo line. I think the UA’s interfaces all share the same preamp topology, input connection type not withstanding, but their converters differ slightly across the line. This is evident in the marketing jargon as well as the specified dynamic range.
The rack units are stated as having 129 dB of dynamic range except for the x16 which has 133 dB. Twin X and x4 have 127 dB, but I don’t see the spec for the Solo/Solo USB. Probably in the manual if you dig, they just are’t advertising it.
Generally these differences are minor, and all the Apollo interfaces are among the best available.
Yes, the Solo can record a stereo pair or up to two mono channels. It has 2 channels of input and 4 of output: Two combination mic/line balanced inputs and 2 balanced 1/4" outputs on the back plus the stereo headphone jack on the front. The HI-Z instrument input (guitar symbol) on the front is normaled to input channel 1.
Whether recording stereo or mono from the S2400, you would typically want to use unbalanced TS cables plugged into the 1/4" rear inputs. That would present the simplest and cleanest signal to the converter.
It is possible to use the 1/4" HI-Z input, or to connect balanced TRS male to XLR male cables to the XLR mic inputs if you want to take advantage of the Unison preamp technology when combined with a compatible plugin. However the signal will still be unbalanced (which may not always be desirable) and careful attention to gain staging is required to achieve the desired saturation characteristic without clipping the input.
Whether you decide togo with the Twin X or Solo will depend on your budget and needs.
Some things to consider:
A Solo could be great for a stripped down live or 2 channel mobile rig, but if you link inputs 1-2 as a stereo pair and put a Unison preamp plugin on it you’d run out of DSP pretty quick. Might have only enough remaining DSP for one more stereo insert plugin while tracking, like tape emulation or compressor. Additional UAD processing can be applied at the mixing stage once the audio is tracked and the Unison/insert plugins are disabled.
A Twin X would allow for expansion via Optical input and has more DSP, but it’s (more than?) twice the price. You’ll still have to manage DSP resources at some point, whether tracking or mixing with UAD plugins. (Its easy to get carried away.)
The Twin also has more options for routing outputs. For example, you could use Outs 3-4 to sample from your computer through the S2400’s inputs with anti-aliasing filters, or because they are DC coupled, send CV generated in a DAW to a piece of external gear.