Just from looking at the Selle Italia SLR Product Page it seems that there is little difference between "road" and "offroad" saddles, as all the saddles listed seem to be recommended for both "road" and "offroad". The same goes for Brooks who group road and MTB in one category. There are various models and I can't see any labelled simple SLR. But if we compare, for instance, the SLR Tekno and the SLR XC, you'll notice that the SLR XC is double the weight (170g vs 85g). I can only guess that the increased weight would give it more strength, and it would probably be able to able to withstand a few more bumps. It would probably also have thicker padding to be a little easier when going over bumps.
Personally I think it all comes down to what actually feels comfortable for you. As well as what fits your riding style. If you have a full suspension mountain bike, you may not need the saddle to absorb a lot of impact. If you have a road bike, and the roads aren't that nice where you live, then you may need something a little sturdier, or with a little extra padding.