People want to turn on their computer and have the OS boot and get out of the way. Adding a bootloader in between is just annoying. You either default it to autoboot X, and then find it frustrating that you need Y, or you set it to always pester you to choose, which is annoying in of itself. The UX experience really hasn't improved ever and kinda can't.
I've been dual booting since fedora 4, and whether it's stock GRUB, or one of the flavours of the month GUI boot loaders, it's the same lowsy experience. I can live with it, but I wouldn't dare inflict it on a new user.
UEFI has somewhat changed it, as I now just default boot to Linux, but from Linux can run a script to set it to boot to Windows on next boot, and then reboot, but it's still pretty gross to use.
USB boot is okay, but perhaps use a USB SSD, standard USBs can be very slow which will give a poor user experience. I also don't think distro is that critical, as long as you stick to the mainstream ones. Make sure it's one that you know well, as realistically, your gonna end up being called for support.