Buzzy vibrators and Rumbly vibrators are different, but mostly in the sensation they give and the noise they make. It's mostly a category for the vibration type rather than strength.
Buzzy vibrators tend to make a more loud, silence piercing noise, and of course make a BZZZZZZ noise reminiscent of a buzzy bee. Buzzy vibrators tend to feel tingly in sensation, so due to this and the noise some people don't like really strong buzzy vibrators. Too much tingling of course and it might start to "sting"....Buzzy bee sting? Bad pun so sorry.
Admittedly I haven't tried many rumbly vibrators, but this is the sort of impression I get.
Rumbly vibrators can be loud too, but they generally make a more quiet, low hum. It tends to sound slightly more muffled, and sounds more like a low pitched Vrmmmmmm. Sort of like a cat purring, or a rumbling game controller. ("RumblePaks"? Too Old?) Rumbly vibrators have a more deep vibration, and they tend to be considered stronger. Mostly they aren't stronger, they are merely felt more deeply than buzzy vibrators. I think that buzzy vibrators tend to be more pinpoint, while rumbly vibrators tend to mostly spread throughout the vibe.
Both vibrator types can come in strong or weak, so being Buzzy/Rumbly does not really decide on it's strength. Also a vibrator can be both buzzy and rumbly, as some vibrators have different settings that have a different sensation. Or a vibration can feel like a mix of rumbly and buzzy. Due to the deeper vibrations I think that Rumbly vibrations are good for internal vibrators. Either vibrator type can be good for external vibes, but I personally prefer buzzy vibrators for clit vibes. Both have good and bad points. Some like both vibe types, some only like one of them, and some people just like them depending on what they use the vibrator for.
o3o Hope it helped, and hope it wasn't too long.