A lot of people get here via that search and it's time for an answer. I've written about supercarrier vs titan before, but that was in the context of deciding which to get. This post is a more general comparison.
In many ways, supercarriers and titans are pretty similar. They have almost the same EHP (33-57 million) and in theory do about the same DPS (8000-11000). The major differences are price, utility and how they do their damage.
Supercarriers, properly fitted, cost about 16 to 24 billion depending on hull price and on whether they are an armor or a shield tank. Armor resist mods cost more, and armor tanks use a slave set while shield tanks don't have equivalent implants.
Titans start around 50 billion (maybe as low as 42 if you get a really good deal), or 65 on the open market. The fittings cost about the same, though shield tankers often spring for officer invunerability fields, which come in 4, 8 and 18 billion isk variations, and titan pilots often use officer cap recharge mods. One figures a minimum price of about 56 billion, with a range about 2.5-4x as much as a supercarrier.
A big deal has been made of the fact that SC do more things than titans. They kill capitals, they kill subcapitals, they rep, they tackle, they neut. Reps are probably the biggest deal -- a supercarrier can permarun 2 capital remote reps with minimal refitting or under lag, making supercarrier blobs annoyingly resiliant under some conditions.
In a fight titans only do damage. They can also titan bridge, which is very important but not useful in a fight.
Titans and supercarriers do about the same DPS, but titans have several advantages.
1. Moar damage. Although the erebus and avatar have fairly anemic damage for supercapitals, the leviathan and ragnarok both do more than a nyx (with a standard fit). With a close-range fit the lev and rag do 50-60% more, which is sort of a lot.
2. Doomsday. With three million alpha, put enough titans in one place and you can kill a supercapital in 30 seconds. Doomsday also volleys dreads, carriers and subcapitals.
3. Lag resistance. Titans' guns and doomsday devices work like any other activated module, and can be cycled in lag. In my tests during extreme lag fighter bombers had to be recalled and re-engaged before they would fire again, which takes twice as long as cycling a gun.
4. Subcapitals. To kill subcapitals supercarriers have to use fighters, which are slow, basically useless at hitting anything smaller than a battleship (normal drones can be used for sub-battleships, but come on) and decrease EFT DPS by 75%. Certainly supercarriers can seriously ruin the day of an incautious subcapital fleet, but in practice they aren't very effective against subcapitals. Titans, on the other hand, can kill any subcapital with the doomsday, which never misses (unless the target warps, so choose wisely). Titan guns are also effective against subcapitals with low angular velocity. You might ask yourself why any subcapital would be stationary with titans on grid, but in my experience sorting overview by angular velocity with a decent hostile fleet on grid will always show a few with low angular velocity -- either they aren't moving, are moving mostly toward or away from you, or are distant enough that angular velocity is low. A good hit by an avatar or erebus does about 30k damage to a subcapital, rag up to 105k, while levi uses missiles and is ineffective against subcaps. Even without lag and assuming the target doesn't warp off fighters take quite a while to do that sort of damage.
Supercarriers do have one advantage, which is that they do full EFT damage to pos mods and sov structures, neither of which titans can doomsday. They're great for defanging towers or shooting sov structures, though useless for reinforcing towers.
Short version, titans are better in a fight. Maybe not enough to really justify spending 2-3x as much as on a supercarrier, but if you've got the isk....