Sigh, these threads must die....
They serve no good purpose but to get N900 & N9 users to fight against one another.
And a time when we need to be supportive of one another more than ever before.

Except that doesn't exist.

There are many regression sw-wise, especially at the upper-most layers, we know that.
But we're finding with each month that work-around/twks/mods nevertheless are appearing.
People were originally doomsdaying, saying it's a lost cause, lets see what things look like in 6mth.

The "biggy" is a fully functioning & unfettered AEGIS open-mode.
I haven't checked in the last 48hrs, but when I did it was still somewhat bogged down.
It comes in "fits and bursts", but I think we'll get there in-the-end.
Software is where the biggest disappointments are, but sw is a very rubbery thing.
What we think may always be a lost cause, can be substantially improved over time.

HW-wise the N900's certainly not superior...
Yes when the N900 was 1st released it was far more aggressive/bold hw-wise.
The N9 should've been way more of a bump, but overall a bump it still is.
Here are the N900's notable hw advantages:

mSDxx
No SD2.0 support means >32GB support will be possible, but it'll never perform as mSDXC should.
N9's built-in 64GB eMMC will perform as well RW-wise as the N900's 64GB mSDH/XC.
If you use N900's two mediums simultaneously that may change, or if you go above 64GB mSDXC.
But the latter is still a pricey proposition....

IR
Up to you, I personally never used it on the N900

HW QWERTY
Ditto here, I've always used BT/USB portable qwerty's.
Far better for serious typing than built-ins.
(but the N900's qwerty was the best built-in I've ever used)

FM-TX/RX(RDS)
RX(RDS) is almost done for N9, & TX (unlike N950) isn't ruled-out yet

Speakers
The speakers can get louder with less reverberation issues.
The actual sound quality is not better though.

No doubt I'm forgetting a few other things, but that's mostly it.
The list of advantages the N9 has is somewhat larger.
I'll leave that for you to do some research on.
But if the above features are critical for you, then the N900 is what you want.