The K420 and K450 are different headphones. The K420 are open headphones more similar to Koss PortaPro, Sennheiser PX100 and Jays V-Jays, while the K450 are semi-closed (I think) and more similar to Sennheiser PX200.
Personally I like the K420. They are less detailed than the PX100 and V-Jays (have never owned the Portapro), but they have a more fun sound signature than both the other headphones. Bass is okay, but slightly more muddy than the PX100 and V-Jays. Mids are not the most detailed, but still great for rock music. Highs are a bit sibilant at first, but it calms down with time and use.
So it depends if you want open or semi-closed portable headphones.