I don't have any direct experience to compare the two models. Though your end product will taste the same.
If the used digital is in good shape, and you are getting a good deal on it, you may want to purchase it rather than to continue putting money in your old BS611. I don't know what types of winters you have, these units are digital, and will not operate under a certain ambient temperature. Also you will need to be more careful of protecting the generator (controller) from moisture.
Just keep in mind during operation, that these units have 20 degree fluctuation range, which should not effect the cooking of most foods.