It's just a tough thing to get right I think. If the CDT is the one trail that you really want to hike, there are people who start with it. If you go that route, I would definitely split it into two years, because otherwise you'll be having more "adventure" than most people care for.
The big catch with "doing the PCT first" is the question of whether life will let you then go on to the CDT. If it's the CDT that really has your focus, then ...
If you do opt to do the PCT first, I suggest that you get Yogi's guide (google it); it's excellent preparation in lots of ways. Also look at various resources on postholer.com, and perhaps peruse the pct-l archives.
Of the three trails I like the PCT best. I have friends re-hiking the PCT this year, and I'll be doing 330 miles of it again soon, hiking southern Oregon with section hiking friends.