DMC, you gotta ask yourself if the advantages of having a DF4 are worth potential seatpost slipping issues, especially if you are a bigger rider (I'm a bigger rider).
I'd suggest looking for a bike with a round seatpost that meets your other criteria. The DF3 fits that bill, if you can find one.
I have never heard anyone say that an aero seatpost made all the difference in the world and things got better. However, I've heard several times people say that their aero seatposts slipped on them.
The point is, the up side of an aero seatpost is negligible. The down side is huge.