Haven't heard the K7xx but I do own both an HE-400i (same as HE4XX) and recently a Sennheiser HD6XX. I'll try to break it down to categories:
HE-400i: Generally flat with emphasized highs, bordering on "sparkly".
HD6XX: Flat with a slight emphasis on mid-highs, and a slight emphasis on the entire low spectrum.
HE-400i: Smooth, realistic, especially with good recordings.
HD6XX: What you'd expect from dynamic drivers. Generally pleasant film-like portrayal.
HE-400i: Little, rather lacking, plenty of detail to feed on though.
HD6XX: Very musical, immersive.
HE-400i: Smooth-sounding due to the planar-magnetic drivers, well-extended on both sides and tremendously detailed to the point it can be overwhelming, much thanks to its treble extension. Similar to one buying a Schiit Vali 2 to get "what tubes are about", I bought these to discover what planar-magnetic headphones are about, and these immediately tell you the story. On the negative side, these are considered entry level on the planar catalog and it's easy to hear why: They lack musicality, and as a result, immersion. Occasionally they can also sound artificial (genre-dependent).
HD6XX: Overall flat, inviting and smooth (in the presentation sense), these have the "this sounds right" sound signature. They are musical, a quality unheard-of in this price segment, and a few songs into an album with these gets you understanding what "immersive sound" is about. The so-called "legendary" status these cans adore is very well deserved, having compared them to my own Shure SE846 and coming out astonished on how close the sound is. On the negative side, the whole-sounding character can get boring. One could also ask for further extension on the bass (considering these are dynamic drivers), and the pleasant, occasionally intimate sound isn't for everyone.
All of the above is obviously very subjective, but if I had to recommend either, I would only need to ask what were your previous headphones.
If you've never listened to something from the $500 territory, the HD6XX is the next step. If you already had a taste of high-end (and don't let audiophiles bullshit you, $500 was and is still, high-end), never heard planars and want something new and exciting, the HE4XX is the right direction, but be prepared to upgrade in a couple of years because they are more of an expo than a whole package.