Not an expert by any means, as I have only used two brands: A Yanmar 5 ft and King Kutter II 5 ft. The Yanmar is very quiet, somewhat light in weight, utilizing a 4 blade tine design. Consequently, the 4 blades cause it to bounce and vibrate quite a lot if the soil is hard. Plus the tines are thin and not too strong, and tines can be broken on submerged rocks and tree roots, etc. The Yanmar digs deeper, probably a foot or more.
The King Kutter is my favorite. It has a 6 blade tine design, is probably 250 pounds heavier, and cuts very smooth, with very little bouncing or vibration. Its a little noisier. 7 1/2 inches is probably nearing its limit of depth. The tines are thicker, wider and very durable. Never broke any of those. But then again, to be fair, I never hit the same things I did with the Yanmar. I suspect however, that the KK tines would not have broken.
If I was buying new, King Kutter II hands down. But, Yanmars can be bought for about $1000. or less, and I think that tines are fairly plentiful on the market. A KK 5 ft can be bought for $1400 to 1600 if you shop.