Well I got to play with one in a Radioshack and I was quite impressed by it the keyboard was very good and the design of the device is quite nice. It succeeds in a few where the n900 failed (personal opinion) like the keyboard on the Nokia n900 was possibly one of the worst I have used (though the n900 did have a better keyboard than the one on the droid), doesn't have adobe flash 10.1 (which the MyTouch 3g slide will get in an update), portrait mode, and some other small things. The Nokia n900 will always have a special place in my heart because of its hackability and geekyness. I will also keep the Nokia n900 to use as an international phone. Specifications for both devices include
MyTouch 3g Slide

Operating System: Android 2.1 with modifed HTC Sense Android 2.2 coming soon
Processor: 600 MHz Qualcomm MSM7227 ARM11 processor
Ram: 512mb
Storage: 512mb of Rom plus 8gb microsd card in the box and expandable up to 32gb
Screen: 3.4 inch 320 x 480 Capacitive Touchscreen
Sensors: proximity, ambient light sensors, and 3-axis accelerometer
Text Input: Four row qwerty keyboard, virtual keyboard, swype, and genius button
Rear Camera: 5mp camera with single led flash and no lens cover
Front Facing Camera: No
Battery: 1300mah
Connectivity: Wi-Fi (802.11b/g), Bluetooth 2.1+EDR, ExtUSB, A-GPS Quad band GSM 850 900 1800 1900 MHz GPRS/EDGE
Tri band UMTS 900 1700 2100 MHz HSDPA/HSUPA (US/Europe) (7.2/2 Mbit/s)
Nokia n900

Operating System: Maemo 5 pr 1.2
Processor: TI OMAP 3430 600 MHz ARM Cortex-A8 CPU
Ram: 256mb
Storage: 32gb plus Microsd expansion up to an aditional 16gb
Screen: 3.5 inch 800x480 resistive touchscreen
Rear Camera: 5mp camera with dual led flash and Carl's ziess optics and lens cover
Front Facing Camera: Yes Vga 0.3MP
Battery: 1320mah
Connectivity wifi 802.11b/g, GSM 850/900/1800/1900 GPRS 107/64 kbps DL/UL, EDGE 296/178 kbps DL/UL, GPRS 107/64 kbps DL/UL, UMTS 900/1700/2100 WCDMA 384/384 kbps DL/UL, Bluetooth 2.1, A-Gps, 88-108 MHz FM receiver and 88-110 MHz FM transmitter, and Infrared port.
There are some things on the Nokia n900 that I will miss going to other devices such as infrared port, maemo, fm transmitter, lens cover, and the stylus. Both devices seem pretty nice though.