I'd suggest the Borg wanted ships assimilated so that these could be turned into further Cubes and sent to reinforce the mighty Borg Collective - and to propel newly captured interesting samples (a category that does not include any of the technology or passengers of the dull old Saratoga) away from the battle site, and to further hijinks in another quadrant and another spinoff show.
It's just that the Saratoga is trivially assimilable intact, while bigger and more interesting ships are going to put up a better fight...
I'm not a big fan of the idea of there having been exactly 40 ships present, and exactly one survivor. For all we know, there were no surviving ships whatsoever (why should the Borg have allowed for such?), and the pods escaped simply because that's what they were designed to do. Surely an escape pod would be provided with the best possible means to escape an aggressor - stealth to defeat starship sensors being the most obvious such means, since superfast warp engines don't appear likely. And by stealth I don't mean invisibility cloaks, I mean the ability to mask onboard lifesigns and pretend to be a piece of jetsam.
It's just that those pods that broadcast a "pick me up" signal were duly assimilated or then destroyed, while those whose occupants were clever enough to turn off the signal beacon survived and remained invisible to the E-D, despite lingering in the vicinity.
As for Picard's "we fall back" speech, those need not be Federation worlds. The Borg apparently got to the Argolis cluster ahead of the Federation, for example, and may have assimilated various uninhabited or inhabited worlds there. At which point the Feds had the option of doing something about it, or of falling back, and they chose the latter, with zero losses of UFP lives or property.
Timo Saloniemi