Want a phone with PC functions? Get Desire. Want a PC with phone functions? Get N900.

They are kind of uncomparable. Read reviews of both phones, go to youtube, etc.

By the way, if you love portrait, you can forget about N900. N900 is landscape only. (Despite some applications which can rotate, but desktop, menu, contacts, etc, the "main" system is all in landscape.

If you don't have landscape, then you better watch some youtube videos about both the phones and then decide. If you have a phone shop close to you (FNAC, i.e.) you can try both phones and decide which one of them fits you.