I found some intresting Information in my electronic gamer and thought i'd post it since there has been much debate on the issue.
Cost
PSP- 250$ Includes 32mb Memory stick headphones,AC adapter, Carrying case,Demo disk and Wrist strap.
DS-150$ Includes Stylus,AC adapter and wrist strap.
Size
PSP- 6.7x.9x2.9 Inches
DS- 5.9x1.1x3x3(6 while open)Inches
Weight
PSP- .62lbs
DS- .61lbs
Screen Size
PSP- 4.3 inch LCD
16:9 Ratio
DS- Two 3 inch LCDs
4:3 Ratio
Screen Resolution
PSP- 480x272 pixles
DS- 256x192 pixles
Available Colors On-Screen
PSP- 16,800,000
DS- 260,000
Memory
PSP- 32 megabytes
4 megabytes DRAM(Plus optimal memory stick)
DS-4megabytes
Game Media
PSP- Universal Media Disc-1.8Gigabyte Capacity.
DS- Cartridge 1 gigabit capacity
(note: 8 gigabits=1 gigabyte so the PSP can hold 14 times the amount of data)
Wireless Communication
PSP- IEEE 802.11b(wi-fi)-up to 11mbps transmission, 16 people Multi.
DS- IEEE 802.11b(wi-fi)-up to 2mbps transmission, 16 people Multi.
Battery Life
PSP- 2.5 hours playing Movies 3-6 hours playing Games
DS- 6-10 hours playing games
Other Features
PSP- USB 2.0 Port, Analog Stick,Can Display jpg pictures, mp3 music and mpeg4 movies.
DS- Touch Screen Backwards compatible with GBA/GB. Build in Microphone.
Hopefully this will help you decide what you want to get .