You have not quoted what you intend to convert/backup. If its DVD yoyr converting then go for the SVCD, playback picture quality will be so much better than VCD.
However, if its an AVI your wanting to convert, then a lot depends on how the AVI was made in the first place (ie. its dimensions and data rate used.) If you play the AVI on your PC and playback looks almost as good as a DVD, then it will convert to SVCD the best by retaining most of the quality thats there. However, if the picture is pretty crappy, out of focus and the file was small, then convert to VCD as there was little quality in the first place and the VCD will reatin what ever was there.
Best option is to convert one DVD to both VCD then to SVCD and compare the results. If you follow the Tutes, they will basically make the best you can do for both formats. You will see the pic quality of both and the time difference it takes to make them. You can now make a decision that best suits your circumstances.