The basic difference is ... 7 years. The RS-11 was introduced in 1956, with a 12-cylinder, 1800 hp diesel engine. The C420 was the modernized version introduced in 1963, with an improved, 2000 hp, version of the same basic engine. (GE 752 traction motors and GE GT581 generator being standard on both models.)
Technically, the biggest change in the C420 was a centralized air filtration system, analogous to those on EMD's GP30 and GE's U25B. This is reflected visually: the RS-11 has a row of square ventilation openings (sometimes covered with louvers, sometimes not) along its long hood, just below the roof. The C420 has one large intake just behind the cab, and then plain sides until you get close to the radiator end of the hood.
Cosmetically, the RS-11 has notches in the corners of its hoods (where the number boards and the sandbox fills are): the C420 has much shallower notches at the end of the long hood (and the short hood on high-short-hood units), and the low-nose versions have notchless noses.
To confuse you more, there are several related models. The RS-32 and RS-36, built in 1961-1963, are very similar in appearance to the RS-11, and almost identical in appearance to each other. (The square vents on the hood sides are grouped into two groups of three rather than evenly space along the hood as on the RS-11: otherwise visible differences are very subtle things like the precise shape of the lifting pads.) The RS-32 had the uprated, 2000 hp, engine used in C420. (Alco had two different systems of model numbering. RS-11 and RS-36 are also known as Dl-701; RS-32 and C420 as Dl-721 and Dl-721A.) I think Alco's idea was that the RS-32 would replace the RS-11, but some railroads wanted lower-power units, and the RS-36 was an 1800hp version (but with some electronic refinements in common with the RS-32). (Norfolk & Western rostered RS-11, RS-36 and C420, so the N&W locomotive photos a George Elwood's marvelous site, http://rr-fallenflags.org/
, might be a good place to practice learning to spot the differences.)
Finally, Alco's Canadian subsidiary MLW produced two similar models, the RS-10 and RS-18. The RS-18 was, internally, virtually the same as an RS-11, but has notchless corners to its hoods: I recently mistook a low-nose RS-18 for a C420 in a photo, though if you look carefully they are easy enough to distinguish. (RS-18 production lasted into the mid 1960s; late RS-18 have the hood-side vents grouped as on the RS-32/36.) The RS-10, produced for a few years in the mid 1950s, had a carbody similar to that of an RS-18, but used the earlier model, 1600 hp, engine used in the RS-3.
I hope that's more helpful than confusing!