I had the original SLO module for years then got the SLO ii and played them side by side a lot.
I think the idea that the original SLO is better is nonsense. Same people who swear that the blue and red channels are different on the original, despite Mike Soldano himself verifying they are identical. I think what trips people up are they set the pots to look the same, and then notice differences. The tolerances for the pots vary, so eyeballing it does not mean they are set the same. The SLO compression and saturation levels change a lot as the gain goes through its range, so people might think one is more open and one is more compressed. If you dial with your ears on SLO I red vs Blue or SLO I vs SLO II, you can get them identical.
That being said, the original SLO can probably be found cheap cheap and it is a near perfect sound, fits right into the mix and sounds glorious.
The SLOII is similar in awesomeness, but then you get that extra crunch setting (which I use all the time) as well as tight and separate EQs.
Either SLO is fantastic and can deliver that 5k tone lol. I really dig it!