I own a Citta but not a Mantis, so keep that in mind. I am happy with mine and it is a fun bike, but did end up making several modifications to it.
There are some questions which you need to answer before choosing between these bikes.
Do you need a folding bike? The Citta is not a folding bike.
How tall are you? The Mantis claims to accommodate a rider up to 6'4". I am 5'11" and had to put a longer seat post on the Citta so it would fit me. So if you are taller the Mantis might be a better choice.
Do need or want full suspension? It adds weight, but is very nice to have on rougher surfaces. The Mantis is full suspension, the Citta is a rigid.
How important is after sales service? The owner of Origami is already answering your posts. With the Citta you are pretty much on your own beyond the actual sale since presumably the Citta company no longer exist or changed its name. Citta is the company name and Gazelle is the model name.