Which upgrade is worth it?

I've been looking at graph tech string saver and ferra glide saddles, are the ferra glides worth the extra cash?
Will a roller nut really help for tuning or will sperzel locking tuners be a better investment?
Fender Highway One HSS Strat
Art & Lutherie Cedar Acoustic
Line6 UX2
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I'd say the locking tuners would probably do more, but maybe get a rolling nut aswell, if you want to be extra sure.

I mean, my Strat still stays in tune with locking tuners, even after some Ritchie Blackmore style trem action!
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Personally, I think proper setup, stringing technique, and a graphite nut is all you need. Check this:
There's several factors, when you're trying to keep a guitar in tune:
- Stringing technique
- proper setup
- As little friction as possible.

So let's attack these possible problems.
Friction - Your guitar has what are called string trees:

The circled metal thing keeps the string from popping out of the string nut. Useful, but they also create friction that prevents the string from returning to its proper pitch after a bend or something. So...apply some sort of lubrication to to the underside of the string tree, as pointed to by the arrow. Use something like Big Bends Nut Sauce, or Guitar Grease. Even pencil lead will work. Simply grind the pencil lead into dust on a paper, and use some of this dust on the point of friction. You can even use machine oil.
The same problem occurs at the string nut. Each groove has a bit of friction. So you can apply the same lubricant there, underneath the string, and that will help. You can also buy the appropriate replacement nut, but the nut should be made of graphite. There's less friction from that material.

Moving on to setup. There's a lot of material to cover with setup, far more than I feel inclined to go into...but I can point you towards some helpful links:

Lastly, stringing technique. Yes, it's possible to string a guitar improperly. I actually mess with my Strats trem a lot, and it stays in tune really nicely. Of course, I have it lubricated at the proper points of friction and have a graphite nut, but I don't use locking tuners or anything like that. The secret? Stringing your guitar the right way! Here's a link that will show you how I do it:
Honestly though, the tuning still slips. On my squier strat, i put graphite in the nut, trees, and the saddles, and it goes out of tune after 3 semitone bends if i really push it.
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thanks forsaknazrael for the helpful links. Aside from tuning though, would the graphtech saddles really help with tone and sustain, and are the ferraglides a lot better.
Fender Highway One HSS Strat
Art & Lutherie Cedar Acoustic
Line6 UX2
No amp ... Help me find an amp
If you string properly, locking tuners aren't really necessary. I use them because it makes restringing easier, but that's pretty far down the upgrade list. Most nuts are fine as long as they're cut and lubricated properly. Roller nuts are a major PITA to install and are definitely not worth it IMO.
New saddles aren't a bad idea, although if you don't break strings often it might not be worth it. If you have a little extra cash, I'd suggest installing a new tremolo. A steel block really effects the tone, and if you can afford a Gotoh Wilkinson or a Callaham unit you'll notice a big improvement in tone and sustain.