I am going to CFObjective, most likely. That would be a definite yes if it weren't for the fact that it's at the tail end of my planned honeymoon :) ...I have gone to both conferences for years and I get decidedly more out of CFObjective. But which is best for anyone depends on their experience and skill level.
If you're new to CF, either from another language or new to web development in general, CFUnited is probably right for you. Although it does have some advanced sessions, by and large it's more junior-intermediate level. Also most of the advanced topics on the schedule will also be at CFObjective (and quite possibly with the same presenters). CFUnited is also really, really big now and any conference that gets to that size starts to lose some of its value (at least for me). They do a nice job with the amenities, food, etc though.
If you're experienced with CF but want to take your skills to the next level, and convene with guru developers in a more intimate environment, CFObjective is for you. CFObjective is no-frills, but packed solid with amazing sessions. I didn't see a single session last year that I didn't absolutely love and learn a lot from.
I'll echo what Rachel says. I run a user group and I make my recommendations to people based on their CF level. Most of the people in my group would get the most out of CFUnited. I also tell them that CFUnited is a great way to dive into the CF waters both from a technology and community experience. They will find lots of sessions appropriate for their skills and they will make a lot of connections to people with greater and lesser skillsets. CFUn(ited) is an instittution. I think it does a great job of getting people exicited about CF and I love turning people on to it.
The past couple of years use of CF has increased at my university and in my group. This has dovetailed nicely with the emergence of CFObjective. Some of our up and coming CFers are moving into frameworks and ready to leverage the level and focus of CFObjective. I hope that users from my group will be hitting both this year. Personally I'm going to try to hit both this year but that's really going to depend on the travel budget. I might go to CFObjective just for Matt Woodward and Peter Ferrell's Mach-ii class. Time (and my boss) will tell.
I've been fortunate to attend both conferences and really enjoyed my experiences. At each conference, I've met very interesting people and watched very interesting sessions.
That said, choosing between the two conferences is easy for me. The smaller, more intimate environment of CFObjective is an easier environment to network and to meet people. Also the level of the sessions is geared toward enterprise level ColdFusion. After 8+ years developing ColdFusion applications, I learn and grow a lot from higher level content.
Whichever you choose, you will get as much out of it as you put into it.