can't speak for simon, but an Alsi contract is highly leveraged, 1pt = R10. If you leave it overnight, a simple 200pt gap will cost you R2000 per contract. And markets have the potential to gap way more than that. CFD's on index futures, I don't know where you trade these (maybe IG?), but a CFD on say Satrix, doesn't carry nearly the same level of gearing, so you can use it for trading trends on a daily timeframe.
Thanks Simon. But if your stop loss is based on a percentage drop / rise, surely the difference in gearing between ALSI and e.g. stxcfd should not influence your decision of holding one overnight and not the other?