I would use the Mini as an htpc, and since I have all my media files on a network drive, 128 GB would do just fine for me. Also, even if it's just a small difference, the SSD should be somewhat quieter and produce less heat?
I think for the Mac Mini the noise and heat should be negligible; I know some people complain about hard drive noise but personally that's not been my experience at all, and I have sensitive hearing and find a Mac Mini practically silent.
Don't get me wrong, I absolute thing SSDs should still be an option for Mac Minis, I just don't think they make sense to be made a standard part for all Mac Minis just yet, due to the cost and capacity. It makes sense for laptops, as there are other advantages (no moving parts, more battery life etc.).
Personally for an HTPC I'd still prefer a hard-drive, even a relatively slow one like the Mac Minis currently use, as it's still more than fast enough for loading and playing HD content, but can hold a lot more of it. I mean, an hour long 1080p episode downloaded online can reach 4gb even with (fairly) good H.264 encoding, so 128gb can run out pretty quick depending upon how long you want to keep files, whether you want to download whole series, at what quality etc.
This is where Fusion Drives are great, or even just any small (more) affordable SSD with a slower, bigger HDD, as it lets you get the acceleration for the OS and apps (so your HTPC can startup and operate faster), without worrying about capacity for your files on the other disk.
But yeah, I don't see Apple switching to SSDs as standard just yet, definitely not with PCIe SSDs being a little more expensive still, as it doesn't make sense for them to make their entry-level machine even more expensive. But the option should still be there for you to choose if you want to, but like I say, my personal preference is a Fusion Drive. Actually, my preference now is for a partitioned SSD + HDD; OS and apps go fully onto an SSD partition, while the other partition is used to create a Fusion Drive with the HDD. This way you can move your user folders to the Fusion Drive, so caches, preferences etc. still get accelerated, while your OS is fully accelerated and your big movie files have plenty of space.