I'd go the DT.
1. They're as common as mud, so parts are cheap and easy.
2. They're still not light-weight, but they weigh about 20kgs less when stripped down to MX spec (I may be exaggerating when I say 20kgs, but still...).
3. Same basic bike as the MX250A, so can easily be upgraded with MX parts (its easy to turn a DT into an MX-replica or better, although it makes more sense to just buy an MX if you're planning on doing that from the start).
4. Typical Yamaha interchangability means that wheels, ignitions etc etc can be used off later models to upgrade.
5. Much better motor.
I get Holeshot Buddy's point about getting a real MXer, but the price difference is massive. A few hundred dollars (up to a grand, tops) should get you a decent DT250 ready to race - it will be miles behind the best bikes in the class, but will get you out there and racing. If you finish last on a DT, it will be your fault, not the bike's.
$1k on a genuine pre-75 MX race bike will get you a f$%ked out basket-case that will take many thousands of dollars before you can put it on the start line.
Depends on what your goals are, really.