Right now I would recommend the Howler, but only until FedSig fixes their known issues with the Rumbler.
FedSig is in the middle of redesigning the Rumbler amp. The original Rumbler amp was voltage sensing and tied into the users siren leads in parallel. This created some issues with inferior siren amps causing them to burn up. The control timer was an external unit.
Then FedSig changed the design of the amplifier from voltage sensing to current sensing and included the control timer inside the amp chassis. The amp was required to be installed with the main siren speakers in series instead of parallel. Other problems arose with the amp not being able to sense current through some sirens complete wail cycle, cutting the Rumbler output on and off during the wail cycle mostly. Some of these problem amps could be adjusted for better sensitivity, most could not. FedSig developed a quick fix for some of the defective units by redesigning the sensing circuit and making it voltage again, and sent out prototype boards to replace the defective ones.
FedSig is currently finalizing the change and production of the new Rumbler amps should begin shortly, if not already. The Rumbler speakers are lighter, smaller and better for tight installs versus the Howler, but the Howler has many more application brackets than the Rumbler. The Howler requires that two wires from its amp be connected to complete the circuit and turn the unit on, the Rumbler just requires a ground trigger (like horn ring). They both perform the same.