Both devices were released last month and they are simply great.But still one of them must be better than the other.
They both have an attractive design.They look great.Wide screen, great resolution, but the N900 has a physical keyboard and that gives it an advantage because some users prefer to use a physical keyboard when writing messages or e-mails.

The size is almost the same but the weight differs in favor for the N900 that has 181g.The HD2 is not that far and only weights 157g.

The display is great at both phones.The resolution is the same 800X480 but the HD2 has a screen of 4.3” wider than the N900 that only has 3.5”.But that’s not a big loose for the N900 because it has 16M colors when the HD2 only has 65k.But not so bad after all.

At the sound section the N900 looses some ground because the alert types are only by vibration and mp3 audio files when as the HD2 has a lot more to offer: Vibration; Downloadable polyphonic, MP3, WAV, WMA ringtones.Still I don’t think that is such an important thing.They both have stereo speakers and a standard 3.5mm audio jack.

When it comes to memory the N900 leads because it has up to 1GB application memory (256 MB RAM, 768 MB virtual memory) and an internal storage of 32Gb expandable up to 16GB using a microSD while as the HD2 has 448 MB RAM, 512 MB ROM and an internal storage expandable using a microSD.
The both devices have unlimited entries in the phonebook and unlimited call records.

The both machines have same GPRS, EDGE, 3G, WLAN, Wi-Fi, Bluetooth v2.1 with A2DP and v2.0 microUSB.The N900 also has infrared port.

The cameras differs a little bit.The N900 has 5 MP, 2576×1936 pixels, Carl Zeiss optics, autofocus, Dual LED flash, video light that features Geo-tagging and video WVGA at 848×480@ 25fps and secondary VGA while as the HD2 5 MP, 2592 x 1944 pixels, autofocus, dual LED flash that also features Geo-tagging and video recording VGA@30fps but without any secondary.

The operating systems differ.The HD2 uses Microsoft Windows Mobile 6.5 Professional and the N900 uses Linux based Maemo 5.I think that N900 uses a better one because it’s an Linux based version and we will see next that it has many preinstalled applications that come with the OS.

The HD2 has a better CPU, the Qualcomm Snapdragon QSD8250 1 GHz processor and the N900 uses ARM Cortex A8 600 MHz, PowerVR SGX graphics.

They have an equal number of applications, almost but they differ a lot.
The N900 uses for messaging SMS (threaded view), Email, Push Email, IM and also Skype and GoogleTalk VoIP integration when the HD2 SMS (threaded view), MMS, Email, Instant Messaging.
The other applications for the N900 are: mozilla browser, MP3/WMA/WAV/eAAC+ music player,
WMV/RealVideo/MP4/AVI/XviD/DivX video player, PDF document viewer, Adobe Flash Player 9.4 and alos has a TV-out.The HD2 uses: Digital compass, MP3/WAV/WMA/eAAC+ player, MP4/WMV/H.264/H.263 player, Facebook and Twitter integration, YouTube client, Pocket Office (Word, Excel, PowerPoint, OneNote, PDF viewer), HTC Peep, HTC Footprints, Voice memo and T9.

They both use a standard Li-lon 1230 mAH battery.
The stand-by time for HD2 is up to 490h using 2G and 390h using 3G.
Talk time goes up to 6h 20 min using 2G and 5h 40min using 3G.
Music play goes up to 12h.

The specifications for battery life look the same for N900 but they are not well mentioned.

So what is the best choice? Well, it depends on what customers prefer and what they will use the phone for.
My personal choice will be the N900 because I have confidence in their company and also in their phones.