Sydenham is useful for descriptions, for
rarity ratings of the coins and for its plates, and not much else. It's arrangement of the coinage was superceded by
Thomsen's rational reassessment of the date of the
denarius and by subsequent
hoard analyses.
Seaby RSC is arranged by
Cohen's "
gens" method, alphabetically by family name, which is an easy arrangement for finding coins (if you know the
gens) but not at all scientific.
Crawford is much better than either of the above. There have been updates to
Crawford's dating of certain issues over the years - notably
Mattingly's article in Essay's
Hersh and some other articles. However,
Crawford remains the
standard rederence for the series. If cost is an issue, try to get the paperback edition or another reprint, as noted above.