It depends on your desk space and your eyesight. How far will you be placing your monitor from your eyes?

Both has the same resolution, 1440p. So the amount of screen real estate will be the same.
The 25" will have a smaller ppi means text and images will be smaller. But the monitor itself has a smaller footprint than the 27"
I'd personally prefer the 27" as I have the desk space and a bad eyesight. 
EDIT-

I just noticed that you said you'll be coding. Have you tried placing a monitor in portrait mode? It's not everyone's cup of tea but if you never try, you'll never know. 

Placing a monitor in portrait is heaven for people who codes, from what i heard. The amount of lines you can see at once is astonishing.
Here's my personal setup.

[attachmentid=4427987]

Both of the portrait monitor are 24" 1200p. The landscape is 23" 1080p.
I decided to skip 1440p and jump to 4k instead in the future.
To me, text & images on the 27" is perfect. Not too big or not too small. I have used one and it's amazing.

I have no personal experience on the 25" 1440p as it's a fairly new in the market.
marfccy have been using the u2515h for awhile. You can ask him about his experience on the u2515h. 

Without youll get used to it. at first it was quite jarring. but my eyes adjusted

youll be hard pressed to find HDMI 2.0 in the market. its still rare. in fact, some shops are still selling 1.3 cables instead of 1.4

but youve got it fixed from the driver side?
depends on your preference really.

for my case, i dont have the desk space to accomodate a 27" and 23" together. so i picked the 25" instead

and boy it didnt disappoint. like i mentioned earlier, the pixel pitch can be troublesome at start, but you can fix it by adjusting Windows scaling or let your eyes adjust. in abt <1 month, the smaller text doesnt affect me anymore than those people who bought 1440p smartphones at 5" screens.

but if you like large screens, especially in terms of purely entertainment (movies games etc), 27" is the sweet spot

Just called Dell yesterday, asked the price for U2715H. Inclusive of GST and delivery will be RM2,119.00. Should I go with the price?

i bought mine from teng_08 at Garage Sales