HFCS is not sucrose-based.  It is derived from starch that has been hydrolyzed to glucose (dextrose) and undergone subsequent isomerization of some of the glucose to form fructose. It can be enriched in fructose by partial separation of the two monosaccharides over calcium-form cation exchange resin. Sucrose is a naturally occurring disaccharide consisting of one fructose moiety and one glucose moiety linked beta-2 and alpha-1, to form a high energy fructofuranosyl-glucopyranosyl linkage. Sucrose is mainly hydrolyzed by enzymes in the upper part of the small intestine, yielding equal portions of fructose and glucose. Thus, metabolically, a 50:50 HFCS would yield the same nutrients to your body as sucrose.  Fructo-oligosaccharides can vary in composition, depending on the source. They can be made by the transferase action of fructosidases on sucrose, in which case they would consist of trisaccharides and tetrasaccharides, and perhaps some higher oligosaccharides, each terminated with a glucosyl unit in a sucrose-type linkage.  There are also other types of FOS which are derived from the hydrolysis of fructans, such as inulin, and tend to be somewhat larger saccharides. Newer types of FOS under investigation also include levan-oligosaccharides, which have beta-2,6-linkages instead of the beta-2,1-linkages found in inulin. The advertised benefit of FOS is that there is evidence that they can serve as prebiotics, selectively enhancing the growth of beneficial bacteria in the gut. Sucrose and HFCS do not act as prebiotics, as far as anyone knows. However, some of the sweetness in FOS can come from residual sucrose and fructose, so that must be taken into consideration, too.