Oh wow, that's a tough call. I love them both! 4DS has some of my favorite Cathe weight work- you can go a little heavier on most exercises, so that's nice. It is a bit less of an endurance workout, or you can at least treat them that way. I love that the leg workout is short, too. You can always throw that one in if you need to do LB in a shorter length of time. CTX's weight work is faster paced with more reps. I also like how Cathe does the weight work in "rounds" in 4DS- you can easily repeat rounds to really kick up the intensity.
Cardio wise they are both awesome. I find that 4DS choreography is a bit more complex (the step workouts) than CTX so if that bothers you, you might want to stick to CTX at first. I guess my slight preference is probably CTX from a cardio perspective- you get more variety by getting more workouts. But the bootcamp cardio in 4DS is amazingly fun, too. So you can't go wrong with either in that respect.
My vote is that you need both! Actually, I think you do need both, but if your focus is on weights, get 4DS first. If cardio is your preference, either is terrific, but for the sake of variety, I would get CTX first.