The selection of detectors has more to do with functionality than just pure depth. Most of the better detectors will all come close to the same depth. Some might do slightly better in the silver and copper targets and at the same time will lack in the lower end of the conductivity range. The opposite is also true, a gold detector is a good example, very sensitive to small gold but not very good with the silver and copper (higher conductors). Most of the detectors on the market are what is considered general purpose and does very good across the board. Other factors involved in detector choice is price, weight, balance, coil selection, service, audio quality, features and sometimes what your friends use or where you hunt and what you are hunting for.
Picking out the right detector for you is much like picking out which vehicle to drive. Car, Truck, SUV, Ford, Chevy, Dodge, GPS, big engine or economy............. you get the idea. Picking out the right detector for you is a personal choice and sometimes you just have to "test drive" the ones that you are interested in and see which one fits your hand and ear best.