I purchased iSyncr and I'm not sure if Double Twist was a purchased app but I have both and use iSyncr exclusively. My complete iTunes library occupies a lot of space so naturally I have a smaller set of playlists(in genres) for my lower capacity ipods and MP3 devices.
ISyncr works by synching your selected iTunes playlists with your phones SD card. I attach my N1 to my MacBook, look into the SD card, find the iSyncr file and double click to open. Window opens up with a list of all your itunes playlists, check the playlists you want and it will calculate available space on your SD card with what you selected(I don't quite remember the details). Click on sync(?) and it takes care of everything automatically. Eject USB device and all is well after the phone and media players rescans the SD card
To be honest, I haven't looked into any other itunes/MP3 syncing app for months so I'm not sure if there's an easier or better yet, a free app that works just as easily. I'm pretty content with iSyncr, nothing flashy, just a simple and easy to use syncing app.