The 43 generally runs the same route as the 42, but with a few time-saving changes. This route only operates mornings and evenings only on Monday-Friday. To be exact, 7:50am-9:15am and 3:20pm-7:00pm. Morning buses begin in Mt Pleasant and terminate at 17th and I (Farragut Sq). The northbound afternoon 43 buses begin at Farragut and terminate at Lamont and Mt Pleasant Streets NW.
The 43 does not make stops at Dupont Circle. When traveling on Connecticut Ave, in both directions, the 43 bypasses the Circle by traveling through the underground tunnel. The two bypassed stops (Dupont Circle South and Q Street stops) will still be served by the 42 bus.
Dupont Circle was always a time killer for the 42. It added the most time to the trip downtown, more than any other factor. The downside to the move is that it will take a little more work if you want to catch the 43 after exiting the Dupont Metro station. You'll have to walk an additional block north or south on Connecticut. But at that point, it runs the same route as the 42, so unless you really, really want to ride the new bus, just catch the 42.
A few minute changes to the 42 schedule: WMATA added a 2:15am, 2:40am, and 3:04am southbound buses to the Monday-Thursday Schedule. They previously only ran on Friday nights (early Saturday morning).