Heart of Tarrasque is a bad Item. I Love it. And It makes your health bar Dauntingly unconquerable. But it's not the only answer to being tougher. A Largely overlooked Item is 
Eye of Skadi. Especially when your character has low armor (under 15). Another note is that if you're already tough from spells via 
Black King Bar Eye of Skadi can do more than a Heart by slowing the attack speed of the opposing Carry by 20% essentially increasing your combat time by ~20% (that time gained just from the slow.) Eye of Skadi compared to Heart of Tarrasque:Builds better than a Heart!
Point Booster is an amazing Item, more bang for your buck than both 
Vitality Booster and 
Energy Booster
Orb of Venom is amazing if you are a melee hero.Costs 175 more gold
offers -335 HP
3.5 more armor. about 6-7% more damage reduction (this percent is bigger if you have low armor, smaller if you already have a lot of armor)
+25 Attack Speed
+25 damage
(if STR hero, -15dmg)
+250 mana
And 30% movement Slow
Note: it can be used with life steal
My 2 cents: Skadi Is an aggressive Substitute for Heart of Tarrasque. It increases damage, attack speed, armor, and slows. This item secures 1v1 skirmishes, and gives you the edge against another right-click hero. It's also a huge help against heroes using lifesteal.
I honestly believe that Eye of Skadi is a bad item. Its build up is poor, and by the time you acquire it, its use is already almost completely gone. It has two functions, stats for heavily reliant stat carries like Viper, and the slow for heroes that are primarily chasers like Razor or gankers like Slark. Now as a carry item it has a hard aqcuirability.
Skadi at best is a luxury item, the only person you can make it a core on is maybe Medusa.
I think it's buildup is one of it's strong points.... It's not a battlefury or mjollnir but it's a far cry from radiance or heart, and on some heroes the early PB+OoV can be quite strong (Slark, Spectre, Cow, Jugg, Riki, PA) all use that combo of items well, and full Skadi is fine on quite a few of them either early or as a luxury (Plus some others like BH/BS/Funstealker/Omni/NS/DK/CK/Alch/Cow/Lycan/Undying/Ogre/Morph/Dusa/Razor/Sniper/OD/Invoker/Silencer/QoP/Luna/Mirana/SF).
It's pretty strong, only issue being that it's a jack of all trades -- heart gives slightly better durability taking the regen into account, Manta or Diffusal are better DPS, Drums can often fill the role of PB+OoV for chasing and mana early, for anticarry basher or bfly is better, for chasing diffusal/basher/SnY/just plain OoV can often suffice.
I do think the slow is marginal in larger teamfights, though if you're a chasing hero it can still shine during cleanup.
Note: the AS slow is a -20, which is typically a ~5-15% DPS reduction, not 20%. It's like shivas -- most effective on low AS heroes/illusions. The MS slow can be really quite strong against melee carries.
It most definitely builds better than heart. Heart gives you health at gold 1100, strength at 4300, and the final at 5500.
Skadi gives you movement slow at 250, health and mana at 1450, +10 to all attributes at 3550, and the final at 5650.
so if I'm building a skadi and you build a heart:
At 4000 gold into our Items, I will have 140 more health, 280 more mana, 12% slow, +10 atk speed, and +1.4 armor. I would no doubt have the advantage if we were to fight before you get to 4300 gold. then after 4300 gold we're maybe even. (except I've had that advantage thousand or so gold ago, so i'm contributing more, and farming better.)
So to sum up:
I never said Heart had a better build up, they both have poor buildups. But for what its used as, its build up is atrocious.
I honestly believe that Eye of Skadi is a bad item. Its build up is poor, and by the time you acquire it, its use is already almost completely gone. It has two functions, stats for heavily reliant stat carries like Viper, and the slow for heroes that are primarily chasers like Razor or gankers like Slark. Now as a carry item it has a hard aqcuirability.
It most definitely builds better than heart. Heart gives you health at gold 1100, strength at 4300, and the final at 5500.
Skadi gives you movement slow at 250, health and mana at 1450, +10 to all attributes at 3550, and the final at 5650.
so if I'm building a skadi and you build a heart:
At 4000 gold into our Items, I will have 140 more health, 280 more mana, 12% slow, +10 atk speed, and +1.4 armor. I would no doubt have the advantage if we were to fight before you get to 4300 gold. then after 4300 gold we're maybe even. (except I've had that advantage thousand or so gold ago, so i'm contributing more, and farming better.)
So to sum up:
It's hard to do a straight comparison here - while the health bar on
Both these important extras are quite difficult to quantify.
Skadi at best is a luxury item, the only person you can make it a core on is maybe Medusa.
But yes, I have seen the argument often, and Eye of Skadi is an excellent item on many heroes rather than picking up a Heart.
I like these comparisons, really interesting.