Should I get The Return of the Manticore?

If you can afford it, you definitely should! Even if a lot of the stuff is available on other records you'll:

support the band (needed to show record companies that they still have a commercial interest)
get new (and good) versions of "Pictures at an Exhibition" (5 Channel Dolby Surround - identical to the one on In the Hot Seat), "I Believe in Father Christmas," and "Touch and Go" (ELPowell), and pieces from their pre-ELP groups: "Fire" (Crazy World of Arthur Brown), "21st Century Schizoid Man" (King Crimson), and "Hang on to a Dream" (The Nice).
There is also a version of "Pirates" which is far better than the one on Works Volume 1, and the unreleased instrumental pieces "Rondo" (live, Dec. 9th, 1970), "Prelude and Fugue" (by Gulda) and "Bo Diddley" . However, nothing remarkable about these last ones.
get a nice booklet with pictures and bios of the group