Welcome, Herra Jirik! Also, welcome to the world of black powder shooting. As you say, both of these pistols will be be more accurate, and for much longer, than the open-frame Colt replicas, and both are very good to shoot. Both are also quite large in the grip, and to be honest, either of them would be a good buy. Armi San Paolo are not well known here in UK, but over in the States, where they are sold as Euroarms, they are very popular in their Remington form, but not so much as a Rogers & Spencer. I have shot both - a lot - and have to say that I prefer the Remington model.
I'm assuming that you can buy black powder, and either have, or can easily buy, the correct ball mould for these pistols - .454" for the Remington, and either .451" or 454" for the R&S. Both will like a load of ca. 24gr - 30gr, in my experience, but half of the fun is finding out what YOUR particular gun likes to shoot best.
As for the grip size, as I noted, both have bigger grips than any Colt except the Walker, although they are still small by modern day standards - we are all a lot bigger than folks 150 years ago. There are, as far as I know, no larger grips available, unlike the Ruger Old Army, for which larger grips are easy to find from Pachmayr [rubber] or Herrett [wood].
Please give my best wishes to the lovely town of Turku, a place I used to visit a lot with my friends from Tapiola when I was younger.
tac