$300 for both?
That would be a red flag on it's own.
XTS are old radios, 20+ by now and many have lead a hard life. They won't do P25 Phase 2. The usefulness of it depends on the band split and it's flash code. Any 20 year old radio that hasn't had a recent visit to the service monitor would be questionable.
APX-6000 at $300 is way below market value. Again, pay attention to band split and flash code. An abnormally low asking price should raise some concerns.
"Pre-programmed" makes me think "stolen", so unless you know the seller well, be wary. At minimum, be careful with a radio that someone may have tried to do NAS on and possibly screwed up. A system admin killing your new radio makes for an expensive doorstop, or a hard conversation with said system admin.
A radio like that for $300, if it's not hot, probably has been beat to within an inch of its life. Service monitor trip would be advised.
For both, consider that the batteries would need to be replaced, as well as the antennas, unless you know for sure they are in good shape (verifiable, not just by looks).
Compare all the unknowns of used radios to a new Unication that will support Phase 2, has a warranty, and I'd not have a hard time making the decision to go with the Unication. No concerns about a Unication having a bootlegged radio ID that gets killed by system manager.