I happen to have both meters (BM789 and BM869s).
They're both great, but I recommend getting the BM789 just because 869s, like Dave already said, is HUGE.
That's the only reason I use the 789 more than 869.
869s uses 9V battery with old-school clip and you need to "peel" the rubber jacket to access it.
789 battery solution (3xAAA) is much better and it works well with rechargeable Ni-MHs.
869s has 500000 count resolution on DCV but doesn't really have accuracy of a proper 5.5 digit benchtop so it is of limited use.
869s has dual displays (bigger and smaller digits) which is useful, for example min/max/avg, DC+AC, dual thermocouple probes (789 also supports two thermocouples but you need to press button to switch).
Both meters have 100kHz AC bandwidth which I found helpful for identifying some problems earlier, potentially. I spotted misbehaving opamps and excessive ripple on switchers before even hooking up an oscilloscope (not that 100kHz is much, still, better than your average DMM).
Also, 869s' beeper is LOUD
. Almost too loud. Might be useful in workshop environment though.
Both meters have decent continuity tester. 789 flashes backlight on continuity and emits short beep on successful diode drop test (and beeps continously on short), 869s doesn't do any of that.