Well, the RCAs are all collected in one nice package, and the sound is great. . . .Really, til you get to the second half of the forties there AREN'T too many small group recordings to get: this was the age of the big band and most Armstrong recordings were Pops fronting a big band. . . .
The GRP Decca cds are really quite good. . . . The sound is very good, the notes are solid, and they're well selected and presented, being pretty complete up to the cutoff date. Definitive has two multi-cd sets that will collect all the Decca masters. . . that's one way to go.
I've some of the Chronogical Classics and they too are great. . . going to have all the commercial recordings chronologically and encompass V-Disks and other items that fall between the cracks of some of the other series.
A decade or two ago I wasn't too crazy about the big band sides and these decades of Armstrong. But over time that has changed: I need Pops of all type and time. Your mileage may vary. . . . B)