Normally, you should go with one large stick rather than two smaller ones. UNLESS the big one is considerably more expensive than two of the half-sized ones.
Stability shouldn't be ANY concern in _this_ decision. If it is, you either got flaky RAM or a poorly designed mainboard and/or BIOS. Any mainboard should run as many DIMMs as it offers slots - if it doesn't, something is seriously fishy about this piece of hardware.
regards, Peter