OS and software
Both devices run on the Symbian operating system. However, while Nokia has based the E90 on Symbian Series 60, Sony has stuck to the touchscreen-friendly Symbian UIQ. Though, both phones come packed with the usual assortment of enterprise applications (office suites, push mail, etc), it is easier to avail software for Series 60 devices compared to that of UIQ, reason being there are more Symbian Series 60 devices around.
Winner: E90

Online experience
While both devices come with full-fledged Web browsers, the E90 is the winner here with its easy to set up default email client. The P1i demands far too many details than necessary – it is a fair chance you will have to go scurrying to your network administrator asking for port numbers and what not before you can really use the device for e-mail. Having said that, both devices match each other in their browsing abilities.
Winner: E90

Performance
The E90 scores over the P1i here too. Although a tad slow to start up (like all Symbian Series 60) devices, it pretty much bucks up along the way and what with its ace features a la HSDPA browsing displaying web pages in its no-touch screen; Wi-Fi and GPS, you're bound to be pleased as punch. The P1i is a wee bit sluggish in this regard. Quite surprisingly, to me, the E90 scored in battery life too, in spite of having two displays (outer panel has a 320x240 pixel display) and the inside unravels a 800x352 pixel, 16-m colour screen). All this coupled with my putting all its multimedia (3.2 MP cam and Mp4 videos) and browsing functionalities to the grind, it didn't die out soon. The P1i's battery drains too rapidly for comfort, especially if one uses the camera a lot.
Winner: E90

Value for money
The P1i is available for Rs 24,995 while the E90's is priced Rs 40,500. While the budget conscious may be swayed by the lower price tag of the P1i, considering the features one gets for the money, the E90 surely holds the edge.
Winner: E90

Final verdict
The Nokia E90 wins by a 5-2 margin. The Sony Ericsson P1i may look and feel better, coupled with a multimedia edge but sheer performance and value for money factors takes the Communicator a step ahead of its counterpart. Mind you, its reign might come to a precocious end if Sony Ericsson develops a better interface for their next P series device.