i just got done with a little bit of the same topic. Here is the link to the thread i started that had alot of good information brought up
OCi or D4i or i450t. mine is not going to be a back up but since it is a watch style i thought i would give a little input.
My choices were between the D4i, OCi, and i450t...i ended up going with the i450t. havnt got to dive with it but feels great, looks great, and played around with it at the house and very user friendly. i was able to get one new in the box with a new transmitter of ebay for $630.
take this with a grain of salt as i am a newbi and also have not been able to dive with my i450t yet, but here is what made my decision for me.
#1- screen is very easy for me to navigate since the dive mode display is broken up with divider lines. my Suunto i had prior does not have the divider lines and sometimes i was translating a number to the wrong information.
#2- i450t looks amazing as a watch IMO. And that was another big reason for me to switch. I live in Alaska so i am a vacation diver only right now and i wanted something i could wear 24/7 while on vacation.
#3- very very simple user replaceable battery on both the computer and transmitter. and i was able to find battery kits for both units online fairly cheap so i can have a spare battery kit for each unit in my Save-a-Dive kit.
#4- was able to get a smoking deal on Ebay for it. cant ask for a better deal than i got. Both the computer and transmitter were new in factory packaging for only $630.
hope this helps out a little