I ran sum numbers as to the difference in armour values, just so you know. @ 388 Armour you have a 79.5% reduction in damage. @ 320 armour there is a 76.19% reduction in damage.. So that extra 68 armour from Guardians gives you only a 3% increase in damage reduction.
Whereas a 1300 increase in health is around 40% increase on your current amount.
Obviously this is your build and these are only my opinions.
Still stand by a good build.

In regards to the comment about no survivability in top lane. Just take Clarity instead of heal and all your problems are solved.

Yeah, I have looked into that before, I think with the sheer amount of MR and Armor Amumu builds he doesn't need hp as much as he can tank primarily everything not to mention is E + his ultimate. If for some reason he does die, the Guardian is something to fall back on. I really don't see this build dying very often. I guess if the team you are playing would call for a bit more HP as opposed too Armor and MR, a Warmogs would definitely be a viable option.

This guide is interesting, but I see a couple of issues. Mainly, you'll never survive top lane with this build. You have no sustain or mana, so an opponent can easily wear you down and you can't really do anything since you're so mana dependent and will oom quickly.

You are also severely overestimating your damage with this build, until level 8, your level 1 E will do 75 damage before magic resist, maybe around 60 after. 2 E's will not do any significant damage unless your enemy somehow has less than 200 health. Also, in today's top lane, most top laners will harass using skills rather than simply autoattacking you, so your E's cooldown will not be refreshing that often, and thornmail will not be that effective (it only reflects autoattacks).

Last thing is that you are rushing all your armor first before MR. This is not a very good thing to do, you should always have a mixed amount of armor and MR in order to tank to maximum effectiveness. This is especially true if you are facing an AP top laner, since your armor won't do anything.

How exactly do you take your opponent down to half hp in one QE combo? most solo tops will just hide behind minions and farm safely. and if you walk up to them, throught their minions, into their zone, try to fight them... how exactly do you get out again? do you expect them to run while you have your abilities on cooldown?

Decent Build!! Amumu is my main tank and our builds are fairly similar. One change I would make would be to switch out the Guardian's Angel for a Warmog's Armour. Personally I see no point in the Guardian as it has a 5 minute cooldown , which by the time you get it, will only be used once (maybe twice)and generally when you get back up the enemy will knock you straight back down (Especially with Darius having a true damage Ultimate that can easily wipe 750HP off a tank). Whereas the 1330 HP + the good HP/5 you get from Warmog's can keep you alive in the first place. Aside from that being my personal preferance, great build. A Tanky Amumu is hard to take down and this build should prove it.
+1