The M5 and M3 are very similar. Although the M5 is slightly brighter, it doesn't make that much of a difference (less than 1/3 stop). The reason for the number change was due to a change in the composition that changed the bulbs from class M (medium peak) to class MF (medium peak, long duration).
One thing to note: although the #5/P25 and M3/M5 have similar guide numbers, they do not have similar light output. The larger bulbs are about a stop brighter, but they are designed for 5" reflectors. The M-base bulbs are designed for 3" reflectors, which focuses the light more, giving an equivalent (or similar) guide number.