This question was prompted out of a short discussion from this question when it was proposed that a upright conventional bicycle is more comfortable than a recumbent trike when riding over forest trails.
The reasoning is that on a upright conventional bicycle, you would just stand up and let your legs be the suspension when going over the bumps. Obviously, this is not possible with a recumbent trike.
So which one is more comfortable?
Comfortable is very subjective. I hate to use a car metaphor, but someone in a trike might find it more like steering a station wagon than a bike when it comes to tight turning. So in one concern, you're replaced an uncomfortable upright posture with a possibly stressful steering style.
I would search for recumbent videos and see how many you can find that show riders sliding sideways and flipping. Three wheels is more stable for an object at rest, but what a trike lacks that a bicycle relies on is the inherent meta stability provided by counter steering. Consider on a bicycle, your front wheel is constantly steering, even if subtly. This gives it remarkable "technical" navigation skill, which MTB riders must rely on.
The length of your wheel base also has an effect. A tadpole trike, with longer wheel base, might make pitch sway over bumps less pronounced. However, you get more roll swap left and right than on an upright, because you have two wheels on the side that are hitting roots at different times.
Trikes have different clearances for obstacles than bicycles. The tadpole recumbent trikes that I'm picturing only have about three inches of clearance under the seat. This is often due to smaller wheels and a road riding design.
The diameter of the wheels affects bounce and shake as well. A 29er will ride a trail more smoothly than a 20 inch kids bike for obvious reasons. If you are looking for comfort, you want a trike with large wheels. This also has an effect on pedaling style - recumbents want a faster cadence to make up for a lack of standing-weight pedaling power. If you are going uphill over roots, you might feel like you are pedaling a lot faster and possibly going slower.
I like riding my Rans Tailwind, it is comfortable, but I notice it's a lot jouncier on gravel paths because it's got 20 inch wheels. I don't get neck strain, but I really don't like hitting roots or potholes with it. If I'm on an upright bike navigating roots, I tend to be looking down and not out at the horizon anyhow, so I don't feel neck discomfort in those situations.
Comfort is subjective. Your setting and riding style has a lot to do with what equipment you want to use.
Update (As per Mac's point) In my experience, which excludes recumbent trikes but includes small wheel recumbent bikes, you are very unlikely to pitch forward if you hit a bump or root badly, so a recumbent orientation is much safer in that respect. The frame sag is noticeable to me, when I strike a root or pothole on my Rans, I notice that I slow down, possibly slide forward in my seat a bit, and my whole body jounces up and down. That also tends to make my steering wobble because I hold onto my (over the seat) handlebars tighter. I presume on a trike, you would feel a roll as one of the outter wheels hit a bump, possibly turning your vehicle towards the direction of the bump as the opposite side was now travelling faster, you might want to practice "steering out" of strikes. If you are riding a delta, you would want to be cautious of forward flipping when striking one rear wheel on a downhill, I suspect, you couldn't lean back to compensate. A one wheel strike on a tadpole could be compensated better by leaning forward towards the strike. (Please tell me to hush if I'm putting my foot in my mouth.)
Is it more comfortable? It sounds to me like it could be but takes different steering techniques. I guess it depends on how much suspension you have for each wheel on your trike, too.
On Youtube there are two-wheel 'bents on serious trails. I have two 'bents. I ride off-road, slightly bumpy, on both. I think the one with 20inch wheels is more comfortable off-road than the 29inch one. The 20inch one has mountain-bike suspension at both ends. That helps a lot. Lots of 'bents have that.
As memnoch_proxy points out, comfort is very subjective. A full suspension recumbent could be just as comfy, if not more, than it's upright counterpart. It will depend also on the severity of the terrain, attack angle for hills (up or down), and some level of rider skill.
I have an un-suspended KMX X-class trike which is a blast on downhill runs through the woods with it's stiff frame and tendancy to get air on jumps, but my backside suffers bit. Conversely I've ridden an HP Velotechnik Scorpion FS (full-suspension) trike on the same downhills and while the ride was much more comfortable, I felt that I was sometimes not 'connected' to the trail as well as with the un-suspended trike. But I'd prefer full suspension on road tours any day.
As with all things: "your mileage may vary". :)
