As I understand it, it has to be one or the other...can't have both right now. Right?
If you like the 5070 more, keep it. If, on the other hand, you have had your eyes on the 5970 and that is the piece you've been wanting to get to, then get it. I find that when I begin to rationalize one vs the other, I tend to screw up and do something that I wish I hadn't. I glance at a piece and am drawn to it immediately or not. Personally, of the two I prefer the 5970 because of its functions and overall appearance. I have a 3970 and very rarely have I used the functions, but I just like the watch period.
This worldwide collectability thing seems to have grown exponentially within the past 20 years and all sorts of watches are now considered highly collectible, but when they were in production...many were not highly sought after at all. Look at the Daytona, Submariners, Milgauss, Explorers, 3970's, 5970's, 5070's, 3450's, cushion shape perpet. calendar/chrono, etc.. There will always another model pushed to collect when the frenzy dies down on the current "hot watch".... so it's a crap shoot at best.
My first Patek was a 3450 that I ordered sight unseen and when I got it, I felt it was too large and too thick...didn't like it at all then the 3940 came along which was smaller and thinner, exactly what I wanted. I sold the 3450 to get the 3940. The fact was I didn't care for and still don't care for the 3450 (because of its size, not appearance) even though it brings 15-20 times the money that a 3940 would bring. I should have kept the 3450 and added the 3940 but I simply could not afford it at the time and I wanted the 3940 more than the 3450.
One thing is for sure, Patek will produce other pieces in the future that will tempt you again and again. They may derivatives of the 5070 or the 5970 but I think you will see something come forth within the next few years that will be just as exciting.
I would think long and hard before you let your watch go, accumulate and add additional pieces, assuming you're young enough to allow yourself the luxury of time. You've got one great piece in the 5070 so be patient and add to your collection.
I'm sure a lot of folks here have experienced the same thing..got rid of this to get that and now wish I had it back. I know I have, but that saying "Good judgment comes from experience... and a lot of that comes from bad judgment" is something I try to keep in my mind when thinking of things like this.
Good Luck!