Both birds are very common summer visitors to Europe. Although they are hardly seen they can easily be heard singing from inside reedbeds.
Both songs are roughly similar in composition with notes repeated 2 to 3 times. If it is the first time you hear the songs and each one is singing by itself, can be a little confusing to tell them apart. You can easily identify the Great Reed Warbler song in this recording. The Great Reed Warbler is much more loud and can be recognized by its croaking voice.
The recording features a Reed Warbler singing in the background and a Great Reed Warbler starts singing at 1.5s and at 11.5s.