Need more info-
how do you use the bike- traffic, highway, canyons, track?
how many miles do you put on a year? <1k weekend warrior, or daily driver >5k
What is your budget?
That being said
if you're going to change it all out anyway, it is a great time for an upgrade
stay with a stock 15t front, maybe get a lightened one.
Get a quickchange rear carier with a few sprocket selections, like an AFAM.
520 is lighter and seems easier to find parts for, but 525 is suposed to be stronger.
1 tooth up front is about equal to a 3 tooth change in back. so get the stock rear gear and the next two bigger with the carier. set up the bike with the biggest sprocket first ith the eccentric adjusted in as far as you can and get proper chain tension, so you have enough chain to change gears. If you go small when you set up, you won't have enough chaine to put the bigger gear selections on.
the 14t mod is the cheap way to lower your gearing for traffic so that you don't have to replace the chain, if you're already changing everything get a quick change carier and stay stock on front.
Low speed/heavy traffic <20mph crap will benefit from a larger rear sprocket (or a smaller front) the 14t change is a big jump and may have you running at a higher rpm than you'd like on open highway. so doing a 1 or 2 tooth larger in the rear is a more subtle way to ajdust your gearing to get the optimal performance for your situation.
I'm sure you'l get plenty of other opinions about the 525/520 options. I haven't had a liter bike long enough to worry about longevity so I'l abstain chain pitch advice.