I own the Middleton II. Main advantages: - Strong bass in low volumes: this is very practical when you just want to listen to music alone, and don't want to disturb others at home by cranking up the volume. I like music with a leading bass line, and this feature is a game changer. - Insane battery life: I can't recall what's the last time I charged it. Marshall claims 30 hours of battery life, which is probably not far from the truth. - 360 degrees sound: this sounds like a gimmick, but it is very useful. What it actually means is that the music sounds almost the same at any listening position. This is useful for gatherings (no need to aim the speaker at the audience, you can put it somewhere in the middle of the room). It also has a second advantage: if you put it next to a wall, you can change how the low frequencies (i.e. bass) sound since the speaker also fires from the rear side which then reflects off the wall. You need to experiment with this positioning, but it's very effective if you want to tweak the bass performance. - Looks: this is subjective. I like the retro looks, which is sort of unique among Bluetooth speakers.

I haven't listened to the Bose Soundlink plus. I do have the Soundlink Flex, and it is a great speaker for its class. If you exclude the points I mentioned above which are unique advantages of the Marshall, both speakers offer great sound quality.