Well, in what stage is your app at? How many users do you have to support? What's the app serving... lots of images or mostly text out of database? How much RAM is your app using with mongrels (check it with top)? What's your hardware setup like? If you're on slices, are they 32bit or 64bit and how much RAM do you have?

Passenger's great if you have a lot of RAM (say over 1gb). If you're on 256/512Mb slices, I'd stick with nginx+mongrel (or thins) since that combo will utilize less RAM and won't swap.

Finally, you should test and see for yourself how the app performs when you load it with httperf or something like that.