Both models are desirable... the 3:4 retangular is the earliest, but the double D just shouts "round CRT!", and provides a larger apparent picture size. I like having models with each style, as the comparison makes for an interesting story of early consumer tv development.
If faced with that choice, I would simply buy both, if condition/price is similar.
Earlier thread showing my 2 sets here:
http://www.videokarma.org/showthread.php?t=269086
jr