I own a Pre 3 and honestly, it isn't a bad phone compared to a new Blackberry Z10. Here is a little run down of the things I noticed between the two: (similarities and differences)
the softwares don't go far from each other. They have the similar "cards" feature for easy multi-tasking and gestures for navigation. The big difference-maker is the OS. BB10 is far more superior than WebOS in terms of apps and features but that doesn't mean that WebOS is inferior. Saying that you don't really need that much apps, this might not bother you at all. Sure, BB10 has BBM and all the other popular apps available but that is just one of the few things the Z10 has an advantage over the Pre 3
A big difference here. A slate phone vs a slider. If screen size bothers you, you won't be bothered by these two as their screens are good enough for the hardware they show off. The Z10's design is a common one, a rectangle slab of metal. The Pre 3 has a bulkier build and offers a slide-out physical keyboard, which I see you love. We will get more into that later. Everything in this category seems tied for me, well except the fact that the Z10 has newer chips and processors than the Pre 3 but that can be altered later on.
The major bit, for me. Virtual vs Physical. Personally, I like the physical feel, but it may be no match to the Z10's near-perfect keyboard. The Pre 3's keyboard, just like mine, can get unresponsive and may have a few defects here and there saying that you're a heavy texter. I have the case of the "double letter problem" in my keyboard. The Z10 has an awesome way of inputting predictive text (a slide up from the starting letter) and its clean look never fails the eyes. This part is very opinion based, if you couldn't tell.
I don't know about the Z10 but my Pre 3 has been with me for a long time and it has been good to me so far. Excluding the minor bugs like the keyboard and some software issues, this phone is very stable and fast.
Homebrew on the Pre 3 is very very very fun. Patches, Internals, Themes, etc. You name it, develop it, then apply and install it. Very customizable to your own liking and feel and it is very cool indeed. As for the Z10, apps can do their job as the marketplace continues to expand as Blackberry continues to grow. In my honest opinion, customization on the Pre 3 is better here.
If I were to put scores in these categories and create a little fight between the two... :
Software: BB10 for it being modern, updated and "in-time"
Hardware: Pre 3 for it having a solid and sturdy body which doesn't break when it falls
Keyboard: Pre 3 for it having a physical one
Customization: Pre 3 for awesome homebrew stuff
Overall Winner and the phone you should get asap imho: HP Pre 3
P.S. If it was the Pre 3 and the Q10 that I was comparing and I was a new user to both, I would definitely choose the Q10.
I wish you well in your very hard decision and I hope I helped in one way or another!