haha...yes "mentally satisfied" is very true - a big weakness of mine. I always want the best of the best even if it doesn't make sense.
Well...you know what you want to do with the SP3 and for that - as others have mentioned above - the i5/ 4GB/ 128GB will most likely suffice. Now, the remaining question is about money - not only what you have on hand, but also an assessment of what you may need in the short-term future. These two will (or should) determine your purchase. I was in a similar situation. While I am in academia, but I am not a student (well, one is always a student, but I am not in the formal sense of the word) and my field is not the same as yours. I bought the 4GB/ 128GB version. In my case, it was not a question of money, but a realistic assessment of what I do and need for my needs in the foreseeable future. In terms of HDD space, I currently use a 64GB MicroSD card, but will be upgrading that to a 128GB card in the near future.