Well to start with I am neither an Apple nor Nokia fan boy, I use what I like.

That being said just like anything in the world there is always an up and down side. Lets face it the iPhone 4G looks pretty "smexy" however the overall OS lacks where Maemo has been doing for years ( even in Symbian S60 ).

The way I see it is;
Apple iPhone 4G -
Easy to use, tons of applications ready to install via store and a overall out of the box your good to go. 3rd party community such as Jailbreaking has a strong going but still limited somewhat by the OS. (Yes I realize recently Android was accomplished on it). Limited on Space, no hot swapping SD cards.

Nokia N900 (Maemo)
Somewhat easy to use if you used a Nokia before, could be a little hard for a Nokia first timer. Applications are few when compared to iPhone but generally better quality due to the close niche of the community. No 3rd party really needed as the phone is for most terms "open" (take that as you will). Not limited really on storage due to SD micro and hot swap.

Just my 2 cents. . . back to my N900 and figuring out why my python app keeps bombing