I'd say that either choice would be a good one. The CG does alot of "good" missions, and especially if you fly the helos, you'll see immediate results of your work (ie, rescuing people). But as mentioned before, your chances of flying FW are much lower.
I certainly won't ding helo folks...I flew helicopters for nearly 4 years, and loved every minute of it. In fact I tried to get into the CG through the DCA program, but I wasn't competitive with my measley 350 hours of helo time. Now I'm flying -130s in the Air Force and I love this job too.
All in all, I think you'll enjoy either service. I wouldn't recommend the Navy to anyone unless they just fell in love with the idea of carrier aviation. In my view, USN facilities are the pits and their aircraft aren't kept up like USAF aircraft (I flew C-21s to USN bases all over, and I saw it with my own eyes...just my observations, not intended to say USN pilots aren't the professionals they are). And to fly for the USMC, I'd say you need to be very motivated to be a Marine first.
But between the USCG and the USAF, I'd say both are services with good aircraft, good missions, and good facilities (it's easy to keep USCG facilities nice since they are typically small facilities). And home life is probably best with both of those services.
As for doing a non-flying tour prior to flight training, don't sweat it. Look at it as an opportunity to understand the other missions within the service. That way when you're out there flying your HH-65 on a mission, you have some idea what's going on down there aboard that cutter. As it was said before, it's usually not that hard to get into flight training, you just might not get there right away. Has more to do with training slot quotas than anything else.
As for the whole notion that someone who enters the military with the idea of one day flying for the airlines is somehow "selfish", go shutup and color. Hell, we all can't serve for 40+ years...gotta feed momma and the kids somehow. The airlines provide many ex-military airplane drivers with a good career post-military service. I won't deny that my goal is to fly for an airline at some point. And if anyone wants to denigrate my 13+ years of service I have at this point in my life because I'm "selfish" in seeing myself wearing an airline uniform at some point...well, they can honestly go f*** themselves.
The fact that this young man wants to give 8+ years of his life to serving his country and earning flying training along the way is much more than the average college kid would ever dream of doing in his lifetime. Kudos to you, and if you want to one day fly for an airline, then that's fine. Just remember that when you wear the uniform of either the USAF or the USCG, wear it proudly and serve our nation well, as I'm sure you will.
Just one last tidbit...if you seriously want an airline career post-military, then go USAF, simply because 90% of what they have are FW aircraft. You CAN get to the airlines flying even helos with the USCG, but that route is harder, and you'd have to find some FW ME time somewhere (probably at your expense). Some regionals will allow some or all of your RW time to count towards total time requirements, but the multi-engine FW requirements (between 100-250 hours of ME time) are standard at just about every airline. You gotta have those. And as for Majors...I don't know of any that will allow any RW time to count towards hiring minimums. So you'd have to go to a regional or corporate job to get the FW turbine PIC time. Just my .02 cents on that issue.