Have you considered biotadoma cupido?
They would be good in a group, they are pretty fish especially with a bit of age on them. They are also peaceful and would go well if you went down the discus route.
If you decide you want one of the thorichthys species, then the finer the sand the better. They are substrate sifting cichlids, much like the geophagines.
However I would not recommend any thorichthys as tank mates for discus. Conflicting temperature and water chemistry requirements.
Thorichthys would be great with livebearing fishes like swords or mollies or fish preferring cooler and harder more alkaline conditions.
If your really torn between what to keep and its just on a coin toss, I would let your water chemistry help influence your decision. Most fish look sharper in thier preferred conditions.
I have indeed been looking at Cupid Cichlids - I really like them too. That's very interesting that you say they could potentially be good with Discus!
I had been wondering about going with the finer sand in the main tank for any eartheaters etc, so that's an easy change. I figured the Thorichthys may be too personable for Discus - if I went that route, they would be the "stars" of the tank for sure.
I think at this point I need to sit down and make an outline of the requirements of my favourite species with temp, ph etc and see what good/bad combinations would be. I naturally want to create the most suitable environment for my chosen species, so that's a good idea - I am having issues at the moment with the chloramine level in my tap water so I am testing to see how easy it is to break it down in an ageing barrel before using for water changes - hoping an established filter in the barrel will work. I have been doing tests in a small empty tank which aren't looking so positive, however tonight am going to be setting up a larger empty tank with larger filter to see if the results are more conclusive in a couple of weeks. Basically, I don't want to go the RO route if I can absolutely help it, and therefore need to kill the ammonia on a scale that I can still achieve large regular waterchanges without simply topping the tank up with more ammonia!

That is the main hurdle with my Discus idea, if it seems like it's going to be banging my head against the wall to keep them healthy, then it makes decisions a lot easier. Whatever way I go, I will still be ageing the water before use, it just may not need to be on such an epic scale.