i'm honestly a bit towards windows (i'm currently taking a microsoft IT academy class and getting my MTA-349 cert) but i understand macs are good computers too and these lil fanboys wanna tell you otherwise
what are you planning to do on it OP? you said you're getting into film editing, so you're probably gonna want to get the mac pro. the extra specs will be nice like you said. i have the LG G3 and have a 4K screen on my phone which i absolutely adore, but you gotta realize that with a 5k retina display comes decreased battery life. also, you won't find many movies in 5k, so they'll most likely look the same as the imac on the mac pro.
basically, it comes down to whether or not you want raw power or a pretty screen. i love 4ks and retinas, but holy hell battery life :(
not a problem if you're gonna be around an outlet a lot though.
if you want my honest recommendation, i'd honestly go for the mac pro despite all i've said above. you'll love the retina display and you can spend that extra $2k on something else. i have absolutely done no reading on thunderbolt storage, so if you're interested in that, i'd probably take it as long as the iMac has specs that can handle the workload you're going to be putting on it.