I've got some new heaters to replace the Marinelands that have been reacalled, but the new ones will stick out like a sore thumb in the DT. Would keeping them in the sump cause any issues?
No., should not cause an issue; that is where I keep mine and no problems so far (hope that wasn't the kiss of death). I think that is where a majority people who have a sump would keep them.
Make sure your heater is fully submersible. I just bought the Fluval M heaters to replace my stealths. But, I can't use them in the sump because the top needs to be out if the water.