Guards of the Iron variety may be more vulnerable (only in a Dwarven context can D6 be considered vulnerable!
), but with two attacks they will of course also lose less combats (especially with a nearby banner) - although you still don't want to lose against S4. Without spear support (in my lists at least) some guys with two attacks are very useful to really cause some damage. Great accompanying heroes in heroic combats for example, look forward to using them with a Shielbearer.
Still, they're too expensive to build an army from, so they're limited in numbers.
Khazad Guard are a good deal cheaper, so can be bought in greater numbers. Tough as a Dwarf with shield, but with more killing potential. (No option to shield though, because of which I'd field them 50/50 with shielded warriors in a warband - let the shields shield, hold off the enemy and keep 'em busy, while the Guards and heroes tear through the lines. Hopefully.) With those limitations and a reasonable pricetag, accessible in larger numbers but still no full warbands, let alone armies.
Long story short: (I like to) take both, but not too many Iron Guard and plenty of normal Warriors too in any case.