Not sure "take down time" is the best way of looking at it. I like the UMP45 because of the high damage.
Now, at short range, it does 40 damage, which is 3 hits to kill. Add stopping power and it's a 2-hit kill. But at long range, it's a 3-hit kill regardless of stopping power.
(TDT = Take-down Time)
Again, whether or not it's the best way of looking at it depends on your strategy. If your strategy is short/medium-range combat, the UMP simply isn't your best option (unless you can't manage the recoil of the MP5K or the TAR, in which case the UMP is an excellent choice, because it's not far behind those two and has lower recoil).
TDT certainly omits other variables, but those variables are largely irrelevant at short distances in the hands of a semi-descent player (namely, recoil and whether or not the weapon is silenced). It's certainly superior to just looking at it in terms of 'number of bullets' or, worse, 'damage-per-bullet', because those tell even less of the story than TDT (because RPM
does play a significant role). What good is a 3-hit kill if it takes "long" to deliver those 3 hits? Suppose you and an opponent are both capable of delivering 3 consecutive shots, your load-outs result in a 3-hit kill, but his weapon has a higher RPM. You are at an irrefutable disadvantage. All else equal (you both see each other and start firing at the same time), he'll take you down every time.
This is why TDT is superior to just looking at the number of bullets needed or the damage-per-bullet, provided the engagement scenario warrants considering TDT (CQB, both players able to deliver the number of consecutive shots required to take each other down, etc).
Again, all of this is really just relevant to CQB and, to a lesser degree, mid-range battles. In these scenarios, a descent player can mitigate whatever recoil disadvantages may exist (such as the TAR and MP5K). But as range increases, this discussion becomes less and less relevant because recoil, accuracy, whether or not the weapon is silenced, and long-range damage play much more significant roles. Here, not only does RPM not help, it can actually be detrimental to an inexperienced trigger finger. Thus,
TDT is then a function of an entirely new set of variables.
At short- to medium range, I'll take my TAR up against anyone's UMP, ACR, FAMAS, M16, etc, any day of the week... and win. At these ranges, its recoil is not a factor for me and its combination of high damage/high RPM makes it superior to all but the MP5K, which will beat me every time (again, all else equal). The AK-47, not so much: despite having a very low TDT, I cannot manage it when firing consecutive shots, so it would make a very poor choice (for me).
At longer ranges, the TAR is not a good weapon for me unless I have the drop on the enemy. Then again, my strategy is based on making sure my engagements are at shorter ranges. In both COD4 and MW2, I won't even engage a long-range target in most cases. The likelihood of me taking down my opponent is low, so why waste the bullets and put tracers out for everyone to see. The reason I still elect to use the TAR over the MP5K (on most maps, not all) is that its long-range damage/accuracy gives me options the MP5K doesn't if I'm forced to engage at longer distances. For my style of play, it's the most well-rounded selection.
But in a nutshell, the reason TDT is such a good measure is that it approximates and illustrates the EV (Expected Value) of each weapon under a specific (albeit very common) set of conditions.
