I´ve got both.
Keep going back to the N900.
If you like to tweak your phone (themes, icons, put widgets and shortcuts WHEREVER you want) then the N900 is your choice.
I can also say that with equal use, the Desire has had more lock ups/freezes and application fails than my N900, however the phone app on the N900 is pretty shite.
Delays in unlocking the screen and having to unlock the phone just to end a call can all be too frequent and annoying.
The N900 email application is great ( I like the fact that I can have multiple accounts that I can see/select all from the same screen, unlike the Desire which you need to switch between)
If you want an internet device that that has a phone, get the N900.
If you want a phone with internet capabilities, get the Desire

Thanks for the Advices guys.. I am still looking forward for some more ...

@jaeezzy & @Kolos
DesireZ & Deire HD is very expensive over here..So i am leaving them... Only Looking for Desire...

@abubakar
Yes tht is my another fear... I am hearing tht Some of the Brand New N900 have Hardware Probz.. Hope Mine will also be good as urs :d

@onethreealpha

Still confusing hehe , Anyways Nice advie , Thanks for tht

I Am Loving N900 through Youtube videos & readings very much... but still a backforce thinking tht " Will it be a Bad decision "

thts y I asked to u guys

Have to be honest with you and say unless you love to tweak your phone and spend loads of time on an internet forum trying to get your phone to function the way you want you are probably better off staying away from the N900. Everyone here will tell you go for N900 but if you went to the HTC forums you can guess what they say.

For a main stream user I think this would be a no contest. HTC destroys the N900. Give the two devices to 100 average consumers to try out for a week that have no bias to HTC, Nokia, Linux or Android and I am guessing like 95 will pick the HTC.

However, the keyboard is important to you which confuses the situation. Desire Z has a keyboard but you have said that is too expensive. What of the Galaxy Epic? Is that available and affordable where you live? That has a hardware keyboard and a pretty amazing one at that, with dedicated number keys.

@bbkshine
For emails, chatting, posting on forums is definitely better to have HW keyboard. You must decide what are your priorities.

I have to be honest with you and say that if you want to minimize expenditures, the N900 wins and it's not even close. Maybe someone will explain how to use their HTC Desire for $15 a month, as I use my N900. Admittedly, it's a bit tricky, but I have more trickyness than money.

It's funny how these threads often have the unspoken assumption:

Money is no object.

From my point of view, Money IS an object -- an important one. So the cost of the phone and the plan actually matter.

Since you can now buy an N900 for $350, its economic advantage is bigger than ever.

Yeah definitely decide your priorities...

But you always try an e bay N900 for...say...2 months...if you don't like it you'll still be able to go with android...and the Nexus S will be out...and at least you'll be the first who will update to 2.3 gingerbread and futures versions...

dead simple...N900.

But even better...wait. Buy a cheap phone and wait for android 2.3, win 7 or meego phone. Will be a great next year

N900.
Because you're here.

You've come to the right place...

Definitely N900 obviously!!!

But seriously, though...

Still go for the N900!!!

HTC phones normally have high SAR levels...

Personally I can't even consider getting any phone before I see Nokias MeeGo phone.

But for me a physical keyboard is a must, so I'd go with the Desire Z.