I find little difference in the "brand" on the can. The only difference might be where it was caught, and then in processing and perhaps the water/oil used in packing. I've never found any evidence that a tuna would taste any different because of who the fishermen sold it to.
I tried the pouch tuna, once, and was not impressed. By the time I add mayo, eggs, apples, pecans, onion, lettuce ... etc. for a salad/sandwich, or use it in a casserole with pasta - it all taste the same to me.
I do prefer tuna in oil rather than packed in water.
There is a difference between light and white tuna ... not because it's chunk or solid - chunk is just solid that broke up into chunks during processing.
The Mercury problem is with Albacore tuna - which is the "white" tuna. Personally, since it's twice the price at regular prices, and I can often get "regular" tuna at 2-3 cans/$1 ... I never eat the stuff.