There is nothing definite about a 2nd edition DVD release at the moment. They might release a "quadrilogy" box or they may not. I don't think there will be a re-edited trilogy collection to come around the same time as the first KotCS DVD release. We'd heard something about it by now.
Even if they do release one, I doubt that apart from a few possible new extras, we'll get one with the most desired thingies that were not in the current set; commentaries for the original trilogy (they'll probably whip something together for KotCS, I figure). One thing that might urge one to buy the current trilogy collection is that they're pretty much cheaper now than they were back in the day.
To me it's always been about the movies, never the extras so I guess I'll just go with whatever first single-DVD release we get of KotCS. They'll hardly be pushing out a re-edited quadrilogy box at the same time as the first KotCS release and I think most of us around end up getting that one no matter the future ones down the so...
This is what I would do based on the info we have now. Of course, anything can come up, but I don't see the $40-60 (or €30-40) a wasted sum right now even if there'll be something allegedly better down road. It's Indy, after all.