Watching Prometheus prompted me to look at the original Alien films again. This Boxset is an excellent way to do it (unless you want lots of extras). It is certainly cheaper than doing them 'On demand' with someone like Sky or Virgin.
All of the Alien films had a choice between the Theatrical Edition and some sort of Special Edition. I think most of them also had a commentary option but I haven't listened to them.
Coming to the Predator films, so far I have only watched the first one. My only comment on this was that the picture quality was noticably worse than the others. However, it was still quite watchable.
I have to say that none of them were that gory and I am suprised they warrant an 18 Certificate. Maybe I have just got hardened to things over the years!
Anyway, at the current price of about £10 for 7 films (of which at least 3 can be considered to be 'Classics'), you can't go wrong.