Yep, the narrower rim will hold a bead better. 15x10's are already too wide for that size tire for off-road use. While beadlocks will solve losing the outer bead, the additional width of the weld-ons will cause other issues. First of all you will beat up the rim lip pretty bad, possibly have more issues losing the inner bead, and the sidewall of the tire is not as well protected.
My previous setup was 38x13-16 TSL's on 16x10 rims (same rim profile as 15's) and I would consistently lose beads at anything lower than 20 psi in the front and 15 psi in the rear. My current 15x8 reverse BS and 39.5x15.5 TSL's have never lost a bead at single digit pressures. Part of this is due to the reverse BS as it makes it harder to lose the outer bead. 78Buford runs the same size rims with 42's and has never lost a bead either running low single digit pressures.