I have worked a bunch with The Grinder and there are three main reasons I like it.
- It's free. I assume from your question, you are only looking at free solutions. There are some excellent commercial products as well, but most of the time, I cannot justify the cost
- It is easy to start up processes on other machines. When really trying to crank up load on a cluster, I need to easily distribute the load out to remote machines. Grinder is great for that.
- The scripts are all Jython. That allows me to easily customize my scripts to programatically tweak my scripts (e.g. randomize certain paths).
I haven't used JMeter in a long time, so I cannot say authoritatively how it compares on points 2 & 3.