I think that there are two big differences between the new and old testaments:
1. CRC2 has a lot of river scale maps, the new testament doesn't
2. The New testament has a lot more runs in it and it's updated
Personally I liked the river maps in the old testament a lot. Most rivers get a map of the various sections including put-ins, take-outs, tributaries, mileages and major rapids. To answer your question you won't get much more out of CRC2, but if you find a copy and have the cash it is a great book to have.
For a little historical perspective as well as some geology, flora and fauna, check out Doug Wheat's, "The Floater's Guide to Colorado," (1983).
It has gem's like "If you are looking for an enjoyable raft or kayak run, Gore Canyon is not for you." and "...Gore Canyon has crushed more boats and destroyed more dreams than almost any canyon in the state of Colorado." But it also tells you who did the first kayak (Walt Blackadar) and raft (Rob Wise, John Adler, and Andy Reich in August 1977) descents and has other interesting information.
Did you know that Glenwood Canyon is made of Grantic Basement rock? Or that if you keep floating the Colorado to rifle you will pass through 7 more distinct layers of rock strata? That the Eagle river was named by Ute Indians who said that "It had as many tributaries as there are feathers on an eagle's tail." - If you like stuff like that, it is worth finding, otherwise skip it.