It's about overall range vs spacing. An 11-32 has a bigger overall range, but a fairly big difference between each gear. A 12-25 has a smaller overall range, but less difference from one gear to the next.
If you like to have your cadence(pedalling pace) just right, you'll probably like a 12-25 more, although climbing will be a struggle. If you're not particularly concerned about your cadence, an 11/32 will give you a tiny bit of higher top speed, and make it a lot easier to climb serious hills.