I've had a two axis EL 400 magnetic scale DRO on my Bridgeport for about 5 years and have been very happy with it. I chose to mount the X scale on the back of the table rather than the front. By mounting it on the back you loose a couple inches of Y travel, but then the slot on the front of the table is still available for clamping large parts. If you choose to mount the scale on the back of the table I would also install some type of stop block so you don't run the shield into the column.
There is a note in the instructions warning not to leave magnets in close proximity to the scales. I regularly use indicators with magnetic bases on the machine, but store them remotely when not in use. In 5 years of use I haven't had any problems.