Grendel doesn't have the best customer service reviews, and I don't feel like waiting for months to get one. It came down to convenience, and the fact that the Randall has a 5 year warranty.
Both the Randall and the Grendel only attenuate about 30 dB. If you wanna crank your amp, you will still need to put it on an isolation bass and, if you're in an apartment or similar, throw it in another room, for serious isolation.
I would say, however, that it cut about 50% of the sound coming off the amp, which is good. Plus, no room interaction. And you don't need to break down and re-set up again, so you will always have that consistency to your tracks if you're recording over any period of time.
