Struggling through this decision too. My uses and needs seem similar to yours. Here's how I'm looking at it:
1. Base i3 SSD is too small. I know many say just run the mini off a fast external SSD (which I already own, so that wouldn't be an added cost, for me at least) or at least move the User folder to the external, but I don't think I want to jump through the hoops of doing that. I also have my two sons' user folders to contend with since they will be using the Mac from time to time, so I've pretty much ruled out the base i3. And, since I've ruled that out, any consideration of an i7/8/128 doesn't apply (again, in my situation).
2. So, at the bare minimum, upgrade the i3 SSD to 256GB at the time of purchase, but then that brings the price up to $999, which is $100 less than the base i5 that already has the 256GB SSD (aside - I'm talking MSRP here - I know there are various deals going on right now on the base i3 and base i5 models as well as deals on BTO models from places like B&H, etc., but trying to take those deals into account just complicates the picture more than it already is).
3. So, whether I would benefit and/or need the i5 over the i3 is a moot point, since I can't configure an i3 were it would make sense to get it over the i5.
4. Doesn't make any sense, for me, to even consider an i7 because I'm sure I don't need the hyperthreading for my uses/needs. Sure, an i7 would have better resale value and could last a bit longer before feeling like it is time for another upgrade, but those are not really important enough reasons, for me, to spend the extra money on an i7.
So, the conclusion I came to was base i5 (with 8GB RAM) or i5 with a $200 RAM upgrade from Apple (i5/16/256). I know I can do the RAM upgrade myself sometime in the future, but is it worth saving maybe $50 over Apple's price for the stress of having to do it myself with the potential of me breaking something in the process (a very real possibility for me!). I don't envision ever having a need for 32GB RAM. If I did, then I would definitely just buy the base i5 and do the RAM upgrade myself in the future since Apple's RAM pricing for anything other than the 16GB upgrade is way out of line.
My previous Macs had 512GB SSDs, so my backup won't fit on the 256GB SSD I'll be getting with whatever i5 I decide to get. But, I've already figured that I'll keep the OS, apps and User folders on the 256GB (all of that wouldn't fit on the 128GB of the i3) and move all my media onto an external SSD (again, which I already own) and either point my iTunes library to the external or install Plex server on the mini and have that point to the external.
So, in my case, the question is immediate gratification of buying in the store today or configuring a BTO with 16GB RAM and waiting a week or more for it.
As it turns out, the podcast that I linked to above was, no doubt, informational and enlightening but, in the end, confused me a bit more before I just sat down and worked out the details, pros/cons, costs, etc before finally coming to the decision of going with an i5.
By the way, I believe the base model configurations and pricing to be quite deliberate on Apple's part and, once again, genius marketing. I believe Apple knew that many people would think the SSD in the i3 too small and then really only have the option of bumping up $300 to the base i5 after realizing that upgrading the i3's SSD to match the base i5 would put one only $100 within reach of the base i5. So for every buyer who went through this same thought process and decided to go for the base i5, Apple got an extra $100 in their pockets.
HTH
