I personally use HP Combi Tanks and have done so for 15 years. I think it is superior to trays and JOBO, (debatable issue for some). They are 4x5 tanks with daylight lids that you use similiar to roll film tanks. I find that the processing is much smoother and uniform over the sheet film then trays.
I did an experiment once, exposed 6 sheets of 4x5 on a smooth card for zone IV, V and VI and processed them in trays normally. Once dried, upon examination, the center of the film had almost 1/2 stop more density then the edges. Apparently when you push the film into the developer, you get a swirl of agitation causing uneven development.
The tanks I find facilitate different agitation times, and compensating development. My standard agitation is 10 secs/1 minute. For compensating its 15 secs/ 3 to 4 minutes.