For air travel I have not seen much of a difference between elong and ctrip. qunar often has a lower price but they come back too often the next day and say sold out- fare no longer available so I have stopped using them.

I find a bigger difference on hotels among these websites. Ctrip's English site will not offer certain hotels which they have on the Chinese site stating there is a restriction that they cannot accept foreign guests. I have booked these same hotels on elong and no problem arriving foreign passport in hand. Pricing is generally though not always the same between the sites. Sometimes one site or the other has a special price so it pays to check.