Both the Yampa and the Green (Gates of Lodore) are beautiful stretches of river and among my favorites! I have been lucky enough to run one or the other (or both) nearly every season and I have to say that it's a toss-up as to which is better.
The Yampa is a longer stretch with nice camps, great hikes and only one rapid that needs a scout (Warm Springs). If you can catch an early season launch, you can boat for 7 days which is a nice spring trip after a long winter (although the weather this time of year gives cause for many to refer to it as the "Dampa"). Since it is free-flowing, not dam-controlled, it's got a shorter season which usually ends by mid-July depending on the snow-pack.
The Green begins with the magnificent Gates of Lodore (you can google it to see what I'm talking about). The camps are all pretty nice - Rippling Brook 2 being my personal favorite and the hikes are great as well. There are three rapids on this stretch which should all be scouted - the first being Upper Disaster Falls - I've seen many boats get hung up below the rapid at the top since it's got a huge rock garden below. At the end of this rapid, be sure to stay left because the channel to the right has an undercut wall which will flip the unsuspecting (trust me, I know - I left an oar and my river map at the bottom of this channel after I flipped). Next is Triplett - there's a camp just above it on the left and it's a hike to scout, but well worth it so you know what's coming. Last is Hell's Half Mile (scout is on the left after a long flat water stretch just after Triplett). Depending on the flow, Lucifer Rock is the "must make" move at the entrance and then, another big rock garden below - once you're past the shelf below Lucifer, get right and stay right until you get past the big rock about 300 yards below, then move to the left. There's really no clear run through the rocks, but the current moves pretty well through there.
That's it for the upper stretches - then you're on to Whirlpool Canyon and Split Mountain. Jones Hole camps have access to great hiking and fishing (be sure you have a Utah fishing license
) and Island Park Camp is the last camp before you enter Split Mountain. The rapids in Split are a bit splashy, but pretty easy to read and run.
Whether you choose the Green or the Yampa, you won't be disappointed. Have a blast!!!