I have both as well. DSL for about a year, PSP fat for about 6 months. I like what both have to offer. Get them both if you can.
As mentioned above, cheap and great looking games on the PSP. More must have titles than general consensus says.
DS has Mario, PW series, Brain Age and knockoffs, Trauma Center, Zelda, FF (for the moment), DQ and the bright dual screens.
I think I have about the same amount of games for both systems, about a dozen each. No bad ones based on recommendations here. Don't have time for marginal/average titles.
My nitpicks on DS: small screens, developers still trying to figure out how to make good use of the touchscreen and mic (there are some good titles but my observation is a general one). Diagonal movement of dpad is bad.
My nitpicks on PSP: noisy UMD, load times, battery life, weird dpad (don't use it at all), screen brightness. It washes out in daytime car environment whereas the DS is bright enough to play in the same environment as long as the screen is not in direct sunlight.
Note that these are my experiences and as always, YMMV.