These two leaf warblers are both summer visitors, though an increasing number of Chiffchaffs now over-winter here, mainly in the south. They look very much alike and their calls are also very similar which is extremely frustrating and often leads to birders declaring that they've seen or heard a "Wiffwaff". Fortunately, their songs are different and there's no chance of mistaking them, which perhaps begs the question why we're bothering with them. Well, if you learn their song, you will be in a better position to develop your identification skills further by noting their behaviour, habits and habitats, and perhaps even learn how to tell them apart from their calls.
The Chiffchaff says its name "chiff chaff", though its German name, "zilp zalp", is more apt. Often, however, it forgets its own name and you'll hear variations such as "chiff chiff chiff" and "chaff chaff chaff chaff chiff chaff chaff". Chiffchaffs arrive (March) ahead of Willow Warblers (March/April), so you'll probably hear them first, especially in woodland areas. They stop singing in June.
I'm not musically minded but for those who are the Willow Warbler's song is described by BWP as a "Lyrical drooping melody of gentle, pure notes in silvery, rippling phrase, comprising several different segments and lasting normally about 3 s. Typically high pitched and faint at start, followed by crescendo to near middle, then dying away to quiet ending, also slowing down." For me, it's a fast delivered, slightly wistful song that peters out towards the end. Willow Warblers also stop singing around June, though I've heard the occasional one singing as late as October. You're more likely to find them in scrub.
Once learnt, never forgotten and both birds' songs become pleasant harbinger of Spring. When you hear one or the other, I bet it will bring a smile to your face!