Hope it helps!
They are very very different! Completely, different! The HD650 requires a powerful amp, sound laid back, dark sounding. RS1i on the other hand, can be driven easily, bright sounding. The HD650 has better imaging and comfort :)
It would take me a long time to retype what I've written about different phones in previous threads. I'll list a few with basic details and my previous posts will have a lot more info.
Here's what I own, my favorite at the top
Yamaha Pro 400-Sounds outstanding with many genres of music unless the source recording is garbage. Capable of exciting highs and subwoofer bass when the song calls for it without messing up the overall FR. This is off a 1 ohm Sansa clip or 0 ohm O2 amp. The bass will probably be a little less controlled if your source or amp has a higher output impedance. Overall I think their FR is pretty well-balanced for not being completely flat. Great isolation. Good instrument separation and works for orchestras. Very efficient and can get loud off portable sources. detachable cable and awesome carrying case. Can be had new from amazon for a little as $159 or as high as $300.
Sennheiser Amperior-Awesome tonality. A bit forward(the fatigue takes a while to set in). Gives a lot of weight to instruments. Can feel very airy at times.
AKG K550-More of a neutral phone. Good highs. Soundstage is good. nice tonality. Very polite.
AKG 701-Sort of a big screen tv equivalent of a headphone. Very neutral but also very exciting and urgent in its presentation. Not a lot of bass but you won't notice it so much unless you just listened to a bassier headphone. Does most all genres decently, nothing is going to feel harsh for the most part.
After these I don't listen to my others a lot. RS-1 is pretty light in the bass with huge bumps in treble on its FR graph. A high output impedance source will change the FR so it has a lot smoother highs, but I'm not necessarily sure this is the smartest thing to do. I think it's very overpriced. DT 990 600 ohm has gorgeous tonality but it's like listening inside a big circle and can get fatiguing fast. UE 6000 sounds real nice but has no treble energy. Momentum is sorta bloated sounding. It doesn't suck at anything but it's not real exciting or good at much either. I liked the AKG 240 a lot but the bass was too muddy on my old gear. I think it should sound a lot better using the low output impedance gear I own now. SR-60 with flats is pretty good. I like my HD 558 better than other Senns like the 650 or 800, but it magnifies static in recordings a lot. I wasn't a fan of the Denon, Stax, or Audeze stuff I heard and thought the Beyer T1 was really overpriced. Didn't like the GS1000 or PS1000 either. I don't know that I'd recommend the HF2 to you. But, I also haven't tried mine on my low impedance stuff. The high impedance gear probably made it sound muddier than it is. Samson stuff like the 850 and R600 are pretty good.
I would say just to listen to what you can before buying and not to spend a lot if you are going to gamble.
If you don't like bass I wouldn't go after the HE-400. Had them for a month and all they could reproduce was bass and treble (Although with magnificent detail and impact(bass)), the mids were also recessed. I can't remember much about their soundstage. You could get a used or on sale HE-500 at 500-600 which are great for a lot of your genres. You would need to spend a lot on an amp to power them though, think schiit lyr ideally. Something for less can be found that works, like my Bellari HA540, but then again it never quite measures up to an amp designed for orthos. Unless you're into instrumentals or crazy soundstage I'd stay away from the K7XX. A pretty flat lineup although they are completely unbeatable for their low distortion (good instrumentals) and they out of your head soundstage. They are bass light though which you might like.