well.. a ROM backup takes a snapshot of the exact system at that time. So, if you restore, you restore ALL stuff or nothing. no pick and choose.
whereas a Mybackup Pro or TitaniumBU allows you to backup specific data and place and reatore back what you want.. apps, data, apps + data, sms/mms, calendar, specific phone settings, etc.
point is.. I am not 100% sure you can flash a rom from 1 phone to another given there might be specific settings that would crack when you try to put a ROM from phone 1 onto phone 2 given things - like phone #'s and ESN's change. Just not sure there.
BUT, if it were me, I would FOR SURE get a most current copy of the ROM just to be safe, I would use MyBackUp Pro and backup all phone settings, data and apps. Maybe even TitaniumBU and do the same. Then, I would copy all of that off the SD card to my computer.
this way, you have several possible backups to piece your data and settings back as they were IF the ROM from phone 1 cannot load on phone 2.
someone with exact experience can pipe in and say if a ROM from phone 1 can go to phone 2.. assuming you even get the same type of replacement phone as who says you even will? if not, another reason all those backups can come in handy to piece your data back at least. A flashed ROM - as your only backup - won't be much help to reconstruct data that can be use on any Droid phone if the ROM doesn't take or you get a different phone.