I think Order of Ecclesia is considerably more difficult and intelligent from a technical game design perspective than Symphony of the Night. One will never defeat the likes of Brachyura, Gravedorcus, Albus, Goliath, Blackmore, or any of the bosses without a sound strategy that involves more than just attacking the enemy. I don't even think you'll get through an area like that. You can go through the entirety of Symphony of the Night without much attention to battle strategy.
Seeing as how I believe Order of Ecclesia to be not only one of the best metroidvania titles, but one of the best Castlevania titles period, I certainly think it's worth it for someone who likes games like it to buy a DS to play that game in particular. However, for whatever it's worth, I recommend Aria of Sorrow above Order of Ecclesia. I think AoS is the most all around solid metroidvania game.
And yet, my personal favorite is Harmony of Dissonance, but that one... my appreciation for it is different than it is for the other two, and on a somewhat different level. I'm not sure if HoD would be as instantly captivating as the other games would be, but it is the most fascinatingly nuanced and enthralling, but due to things that are somewhat abstract. I'm not sure if features like that would connect with you.
I get exactly what you say about everything.
My favourite game ever is Portrait of Ruin, but if I had to be objective, I'd say Order of Eclessia is the best MetroidVania, I guess, and best Castlevania even, though I like PoR better.
Aria of Sorrow is perfect in my opinion. An argument can be made of it to be the best MetroidVania.
About difficulty I think OoE is the 2nd hardest MetroidVania title, because Circle of the Moon proved to be really, really hard to me. There's some areas I couldn't complete. And you can't level your way through the game in CotM.
And about Harmony of Dissonance, there's a strong sense of atmosphere and (precisely) fascination about that game that only few people seem to get, including the soundtrack. I hated it at first because it didn't sound like CV to my ears, but I tried to understand it for what it is, and in the end I really liked it.
Everything gets together perfectly in HoD for what it tries to be.
NEVER beat Albus mode
Neither did I. I could never beat the final boss.