there are many ways how to boot into a particular system but none of them is as easy as holding a key after start.
like macarone says you can pause the search on boot manager (unlinke in the controlpanel before rebooting) but it still includes waiting time, several clicks, and the risk that you sometimes choose the wrong OS by accident.
a reboot command, no matter how it is triggered (keyboard, finder menu, apple script, console...), will always boot into the currently chosen startup folder as set per PRAM.
for most people rebooting from X to X is the lesser case compared to rebooting from 9 to 9, so your best bet is to always choose your (main, if you have many) 9 folder as startup folder and then remember, that if you want to boot (or reboot) into OSX or linux, you have to go via the boot manager.
OS9 = default case, OSX = hold alt. easy to remember.
now what you could do when your machines doesnt support the boot manager or when you want to control a reboot by remote, no idea.
(some machines will also support holding D to boot from the very first partition. or to hold X to boot from the first partition with OSX. but which one is that? ... and it can easily stop working when your disks and volumes setup changes.)
what would be a neat solution to easily choose 1 out of 2 systems would be that you
1. always had the list of currently available boot folders ready somewhere
2. then it should show up somewhere in the finder menu
extras
/restart from disk 1 folder 1
/restart from disk 1 folder 2
but what would you save? now the search for current bootable systems happens everytime when the finder is loaded.