Now that I have gotten a flavor of WP7.5 on the Lumia 900 I can say that if the N9 was polished at its current set of features without any bugs would beat the Lumia 900 hands down from software features to included accessories and overall speed and responsiveness and multitasking.
The stock browser "Internet Explorer" may win on the Lumia 900 but because there are more options on the n9 (opera/firefox w flash) the N9 has an edge here. The IE is fast and good at rendering sites faster than n9 browser even on the same wi-fi N network sites loaded almost twice as fast on the Lumia. I used latimes.com desktop page as an example.
Battery on the Lumia with >1800mAh is a beast. LTE on the other hand is a hungry lion. But using Wi-fi for long periods for browsing doesnt move the battery much. N9 lacks in this, i can put the n9 in LP mode in a few hours browsing even on wi-fi.
Maybe because the 900 is a new phone there are just not many features that you can customize and the software feels basic, like the iphone, but whatever features are there do work. I can only change the color of the tiles, I cant even pick any color but i haven't complained about this on the N9 mainly because i like the layout and there are plenty of settings. Application settings are just minimal, Data usage tools are non existent. I found the back button to be confusing (especially when you close and application and go back to it, e.g. you browser may have history but you cant access it from the back button if you closed the browser) and got lost inside applications just like symbian. E.G. Inside the music player, i could not stop the music from playing easily. I found the quickest way to stop was to lock/unlock and press stop at the unlock drape. You cant quiet an incoming call from the call screen. No skype. Nokia maps do have traffic. The local scout feature is cool, but i hate that every applications want to know your location. Why does ebay need to know my location? more importantly i dont want to tell ebay my location or anyone else who want to collect it and use it for their purposes.
The hardware though is superior than the N9 in many ways, the speaker is very loud and the headset is clear and loud. The screen has a similar resolution but bigger, lot less strain on the eyes. LTE vs HSPA. LTE is fast very fast. Its empty in the States with so few phones available, use it while you can. The screen however is not curved like the n9. its flat and doesnt have the low power feature. There are not headphones or even a rubber case included with the Lumia.
With all that said, i believe that the N9 has a gem in its multitasking, I have not yet seen a phone that multitasks better than the N9. Including the Lumia, iphone and Android. The multitasking pane is the best way to multitask. I also love the feed page on the N9. I just love the 3 page layout of the N9 with the swipe feature over any other combination of home screens with dedicated buttons. I would not give up my bug ridden N9 for a lumia 900 despite of availability of many popular american apps.