I ran a 3.1 for a while some 5 or so years ago. It was a mid level Onkyo 5.1 reciever with Pioneer LCR. I had the dual 5.25 center & a couple of 3 way towers that I recall having a tweet/4 inch/7 inch woofer. I was glad I went with the tower over the bookshelf, but I didn't own stands and with those factored in the price was much closer for my low end pioneer's. I had a darn nice Boston Acoustics 10" sub that retailed as much as my reciever, got a great deal.
I went with the above when I got sick of the weak sound from a cheap HTiB.
To answer your question, if you already have the stands & don't have an opinion about the look of stands vs tower, save the money. If you planned to not run a sub, I'd say go for the 75's, but with a sub & the real LFE crossed out of the fronts, it should work. The room size plays into it as well, are you sitting all the way back, mid room, odd position?
Like mentioned before, it comes down to you. But, looking at the facts (own stands, running sub) I'd say go with the bookshelves and save a couple hundred bucks for the next itch.
Sorry for reviving the dead.
Always a student, never a teacher.