Assuming this is an indeterminate cherry plant:

If you have the space and the support for a bushier plant, you can leave it. It will produce flowers and fruit an increase your total harvest over the season. If it's going to cause a problem with being too dense (i.e., foliage too squished together for good airflow, which can create disease-promoting conditions) or hard to support in the space you have the plant, then you can prune it off.

Personally I've never pruned/suckered back a cherry tomato plant. I do "sucker" or semi-sucker my regular tomato plants (I stake them) to produce fewer but slightly larger tomatoes. Pruning a cherry plant will have no effect on tomato size but will reduce production. For your situation I'd give the extra vine more support via staking or a cage on the side.

Depend on how large is the stem, why not cut the sucker and try to root it in water to get another tomato plant.