I have a Solo 3rd Gen and a 2i4 2nd Gen.
No difference in the DAC section to my ears. Both great.
The ADC of the 2i4 is good enough for vinyl transferts.
No surprises. The first-gen 2i4 was based on a CS4272 codec plus an extra stereo DAC (AK4384, interestingly), so I assume the gen2 will be similar. The Solo gen3 uses a CS4272 as well (like the 2i2). Gen3 is the first to really let the codec flex its muscles, well the ADC side at least, though DAC dynamic range is up slightly as well.
The CS4272 ADC part is pretty much a CS5361. Digital filter specs suggest the following:
A sample rate of 48 kHz is needed to hit 20 kHz of aliasing-free bandwidth, so I'd recommend that at the minimum.
96 kHz may be slightly better-behaved than 192 in some cases - stopband extends to 3fs = 288 kHz vs. 1.34fs = 257 kHz, plus -0.01 dB point is at .43fs = 41.3 kHz vs. .105fs = 20.2 kHz.
Best-case aliasing-free bandwidth for this chip is ~42.2 kHz at 192 kHz. So you might record in 192 kHz and resample to get a solid 96 kHz recording out of it. 96 kHz is good to ~30.7 kHz.