2015 Mazda 6 Skyactiv 2.5L - due to the tax it is usually more likely you'll see high cc variants cheaper than the lower cc variants, in this case the 2500cc is cheaper thanthe 2000cc. A few things to note the 2500cc gets which the 2000cc doesn't get.
1) Power = 153hp @ 6000rpm / 200nm @ 4000rpm vs 185hp @ 5700rpm / 250nm @ 3250rpm.
3250 is really low so this car is really itching to launch itself from every traffic light
2) Kit = Gun Metal color, 19" alloys, i-ActivSense suite of active safety systems, LED lights front and back, keyess entry, sunroof and paddle shifters.
3) Head unit = this variant gets an updated display, 11-speaker bose sound system and navigation (the regular one gets speakers and the old head unit)
Of course when you bring it to service the slightly bigger engine is going to need abit more engine oil but that is about it, alot of the other parts are shared with the 2000cc variant so unless exceptional engine failure you're not going to really need to find specific 2500cc variant parts. The great thing about Skyactiv cars is it gives you hybrid like fuel economy without having to be a hybrid or lugging around a battery.
as i was looking for a brand new car, how much of a disadvantage would it be to get a used car? sorry if this is a dumb question as i am total car noob.