You can do a little more with products such as limit to certain groups, associate with other purchases, add various renewal options, feature, do licence key integration and a fair bit more. Subscriptions was built to simplify this process, and just offer a subscription model of subscribe/upgrade/downgrade and add to user groups when this happens. They are also presented on their own subscriptions page, which is more built for purpose.
Generally, most customers will find subscriptions will suit their purposes better than products. As a general rule, I would say unless there is something specific you cannot do as a subscription, then It's a subscription you would be using.