I own and have used both. They are both excellent lights with plusses and minuses.
Dive Rite 500 LED
Pros: * Operates on 8 AA batteries, these are easy to find no matter where you are.
* Long Burn time. Something like 12 hours at full power and then dropping down in power
up to 30 hours
* Equivalent brightness to Dive Rite 10w HID. (My buddy dives with the HID and our lights
were pretty evenly matched)
* Price - Significantly cheaper than the 21w Salvo.
Cons: * Not as bright as the Salvo 21w
* Light head is actually larger than the 21w Salvo.
* Twist to turn on/off rather than switch.
* Corded option allows you to use just like any other canister light, but you need to buy
either the goodman handle or the elastic handle separately.
Salvo 21w LED
Pros: * Lights up the cave/cavern like the sun..ok not really the sun, but is significantly brighter
than the DR 500 LED.
* Smaller light head than the DR 500 LED.
* Switch to turn on rather than twist.
* Excellent battery life. Option of 4+ or 6+ hour canister.
* Comes with both Goodman handle and elastic handle.
Cons: * Price is significatly more than the DR 500 LED (almost 3X more).
* Battery cost is much more than AA's, so if you want an extra, it's gonna cost you.
* Although run time is good compared to HID's, it is still low compared to DR 500 LED.
(This can usually be a non-issue, as long as you plan correctly)
To address a few of the other comments:
* I've never had a problem tucking my cord behind the DR 500 LED canister.
* You don't "need" a tremendously bright light for cavern, but you will be able to see a lot more.
* Renting is a GREAT suggestion, so that you can try out a number of different options before you buy.
Finally, if you have the money, one suggestion to look at is the Salvo 12w LED with the cord and handle. It is definitely brighter than the DR 500 LED, and costs about 1/2 of the 21w Salvo.