A semi-recumbent tricycle offers features that are not found in any other type of tricycle or bicycle. The riding position is more relaxed than a traditional tricycle and they offer a seat back for more support. There is nothing like being able to sit back in comfort while riding and still get some great exercise. Since it is a tricycle it has a more stable platform than a bicycle so balance is not an issue and you can ride as slow as you like with no problem.
The design if a semi recumbent is not as low as a true recumbent but still lends itself to easier and more energy efficient pedaling. The pedals are positioned forward of the seat post and the seat-back actually aids in pedaling as well. A little back support can make a significant difference in comfort and is a big hit with riders that have back problems.
When compared to a standard tricycle they offer enhanced stability due to the lower seat position and subsequent lower center of gravity. The riding position is not as low as a recumbent so the rider is still easily visible to other vehicles. The design adds a little to the overall length but the difference in handling is not noticeable.
Of the two models shown below the Tr-rider semi-recumbent may be a bit easier to mount and dismount since the seat is higher than the Mission design.