The most inexpensive models have very small screens and little extra information can be displayed. I selected my unit, which is a mid-priced unit, because with my failing eyesight, I could still see the screen even at night. If you have good eyes and want basic information, the most inexpensive units will do fine. The biggest difference in price may be the antenna. This is a confusing issue which costs big bucks.
All units can see the satellites with an unobstructed view, that is to say, from a bass boat.
The only reason for a separate antenna is if you want your unit to operate with something over it's view, like from the cockpit of a cruiser. The extra money is for the amplifiers and receivers to drive the weak signal from the antenna to the unit. This will make your navigation more precise, if you have the recommended 34 foot difference between the main antenna and the differential antenna. Again for the purist, you can divide the wavelength and place the differential receiver at a binary length point with reduced effectiveness. From our standpoint, a waypoint is a location or spot, and a route is a line between two or more spots.