Hi Emilio,
I was in the same emotional state as you, but in my case it happened when Notion for iPad was released about 18 months ago. As a longtime Apple customer, I had been wondering why I should buy an iPad in the first place. I had my Mac and my iPhone, and I didn't need yet another device. Well, Notion became my excuse.

Now I use the iPad more than my phone or my laptop.
My first iPad was an iPad 2 with 64 GB storage. I have to admit I was slightly disappointed with Notion on it. Especially the limited number of instruments the processor could handle without stuttering annoyed me. When the iPad 3 came out, I thought it was too early to update, but when the retina iPad 4 came out a couple of months later, I couldn't resist and bought it, again with 64GB. I was able to sell my iPad2 to a friend, which helped with the finances.
Since the Notion app has been improved in so many ways, I now use it more and more, but always as a "little brother" to my desktop Notion program. I do all the detailed editing on my iMac and use the iPad mostly to check things when I am away from my office, and to make simple changes. I love that the iPad app will play back even the things you cannot yet create with it: Instrument changes on the same staff, complex repeats, and so on.
If you don't plan to store a lot of full length movies and such, I think you would be alright with 16GB of storage. I make good use of Dropbox, where I store all my current work so that I can get to it easily, both from my computer and my iPad. It also cuts down on the amount of storage I need on the iPad, since everything is "in the cloud."
I don't have any experience with the iPad Mini, but for my rather clumsy, big fingers, and my middle-aged eye sight, the regular iPad is already small enough. If you have the chance to try out the two models personally, you should be able to decide which model would suit you better. By the way, I believe the "big" iPad has a slightly faster processor (and more RAM for samples) than the mini. But I could be wrong.
With best wishes,
Thorrild
(On vacation, with only my iPad!)