First I'd question your CSR saying he can get you HR24's. Unless something has changed in the last few days I've always been told that CSR's can't specify which HR2x you get, could be a HR24, could be a HR20.
As to what I'd keep, if the HR20 is working I'd keep if over a HR21 since the hard drive size is the same.
As to what I'd keep, if the HR20 is working I'd keep if over a HR21 since the hard drive size is the same.