As with all great ambiguous and useless responses, my answer is "depends"! 😁
OK, it really comes down to what's important and running a creative based company I would guess you have a high dependency of high availability and high volume data storage. That alone makes a Mac mini only solution deficient. A Mac mini will always need its drives changed to either SSD, which are small and expensive, else enterprise HDD which are still limited to 1TB. As an example, going with a 2 x Seagate Constellation.2 2.5" 1TB drive configuration will add $400 or so to the base price of the Mac mini. That $400 into your Mac Pro can buy a boatload more 2x Seagate Contellation ES.3 storage. You can compare Western Digital or other manufacturers but the numbers run more or less the same.
Our sticking point always came down to storage where a Mac mini could never do it alone and once you added a high volume, 4 drive (to match a Mac Pro) solution, we always found the Mac Pro to be the more economical (both financial and ecological) solution especially seeing as we already had it.
Your milage will vary based on how much data and how you wish to use your data. Consider it without considering the implentation. Once you know what you want to do, finding the right solution that suits is relatively easy.
When it comes to portability, we're finding network home directories with mobile home sync from our Mac Pro the way to go. As we've stipulated a home directory size of 160GB, everyone fits into a 256GB SSD, purchaseable in ANY Apple offering today. None of our project run to any massive size and if required, external storage is available via software encrypted 64GB to 256GB USB3 thumb drives (we're finding the Kingston HyperX 3.0 to work well). Why not an online solution? Rather simple really. If it's "that big" our Internet connections can't support it so why bother trying?! Most times one person is handling things so if the current version is "out" then generally, who cares. If it's taken out we just change the folder label colour to red (we use that for "don't change" or at a minimum ask questions).
Remote access is limted to mail, calendars, contacts and messages (Jabber) and stuff that's, well, easily managed by a standalone Mac mini. Which includes our Netboot, NetInstall and NetRestore images. Both our servers run the same Open Directory, DNS and DHCP setup.
The trick with any good, efficient and economical solution is to fight the battles you can win and to pick the battle that will make the most difference.