Both approaches solve the same underlying problem — the machine cannot stand where the work is — but they solve it in opposite ways, and they are not interchangeable.
The distinction
A long-reach front keeps the machine on firm ground and extends the tool to the work. An amphibious undercarriage takes the machine to the work by spreading its weight until unstable ground will carry it, or by floating outright.
When long-reach is the better answer
If there is a bank, road or hardstanding within working radius, long-reach almost always wins. It is cheaper to buy, travels as a normal machine, needs no assembly, and its undercarriage does not run in silt. Canal and riverbank desilting, pond edges and roadside drainage are typically long-reach work.
When amphibious is necessary
When there is no firm position within reach — the middle of a lagoon, extensive marsh, a large tailings pond, deep peat — no amount of reach solves it. The machine has to go out there.
The middle ground
Some sites are genuinely ambiguous, particularly wide channels where the far side is unreachable from the near bank. Working from both banks with a long-reach machine may be cheaper than one amphibious machine, if access to both banks exists. If it does not, that settles it.
How to decide
Measure the distance from the nearest ground that will carry a machine to the furthest point you need to work. Compare it against long-reach working radius at usable lifting capacity, not maximum reach. If the work falls outside that, you need amphibious equipment. If it falls inside, you probably do not.