Okay, so here's the deal. The great thing about a two-piece toilet is that a manufacturer can put many tank styles on the same good bowl, changing appearance without changing performance. So, today, there are only three (well, for purposes of our discussion anyway) original Drake bowls: the ADA height elongated, the elongated regular height and the round regular height. The 1.28gpf version (E) and the 1.6gpf (S) version both now get furnished with the Eco bowl, the one that is designed to run well on 1.28gpf. (C744E, for example. They no longer make a C744S bowl.) So a CST744S toilet is an ST743S tank with a C744E bowl, and a CST744E toilet is an ST743E tank with the same bowl. The 743S tank and the 743E tank use exactly the same porcelain tank; Toto just puts different guts in each to dispense the appropriate amount of water per flush.
So, how do you make a new-looking toilet without changing performance? You build a new tank that looks different, set it to dispense the same amount of water, and put it on a proven bowl.
Enter the "Transitional" style ST744E tank. Put it on any original Drake bowl, and add an "N" (for "new tank") to the toilet model number, and you're in business.
Hence, any "N" Drake is going to work the same as its counterpart with the old-style tank.
So, the real question you want to know the answer to is whether the 1.28 gpf original Drake works as well as the 1.6gpf Drake, because the N tank original Drake will work the same as the old tank original Drake. And the consensus on here is that it does. I think the "E" bowl saves some of the water by having a slightly smaller water spot than the now-discontinued S bowl did, and by tweaking so it refills without wasting any water down the drain, so all the savings doesn't come just from putting less water into the flushing action. One member here has both, and he swears that if you didn't know which was which, you couldn't tell the difference. The MAP rating numbers bear this out as well -- both toilets have ratings over 500, which Terry indicates is the maximum meaningful number -- anything above that (and I think the original Drake is 1000) is really not meaningful in everyday use.
So, I would say that if you like the look of the new tank, go for it. You can put it on the ADA-height bowl (CST744ELN) or on the regular height elongated bowl (CST744EN). We don't normally recommend the round, unless you have a particular architectural reason to install it (door clearance, etc.) because most men find the elongated (in any brand anywhere) to be more comfortable to use.
The real issue with 1.6 vs 1.28 is how well the bowl is designed. That Drake bowl works very well on 1.28, and dumping a little more water in each flush isn't going to change much, just as holding the handle down on your old toilet and adding and extra two gallons per flush didn't make it flush any better. And, at the end of the day, if you're really in need of 1.6 on every flush, Korky makes an adjustable flapper that you can drop in there and set to add additional water per flush. But you won't need it.