Definitely the 27 cog will be easier to pedal than the 25. The cost of going from the 25 cassette to the 27 is you lose some of the finer steps from one gear to another, but that may not matter to you.

Going to a 30 might indeed require a "mountain" derailer but some road models will work. Give it a whirl (you'll need to adjust the "B" screw so the derailer pulleys do not hit the largest cog). If you end up going that route and find that you need a new derailer, be advised that replacing your current road derailer with a "Long Cage" version of the same road derailer will probably not work . . . you need one with a larger parallelogram and these are almost always designated as "Mountain" or "ATB" derailers. I run this kind of setup; 53-39 on the front and 12-32 rear. Quite nice for steep climbs.