I would readjust the rear derailer after changing cassettes, even if the size is not different. This is probably not necessary, but a quick realignment only takes a few minutes, and I've found it really helps with the shifts.
It's also really easy to do, hit up parktool.com if you need how-to info. Learning to do it yourself is well worth the time.
As far as 25 vs. 27, it's not going to be a huge difference. If you are just barely unable to spin your 25, then the 27 will probably do it for you. But, if your legs are totally giving out on steep climbs, and you do a lot of them, then I would possibly consider a compact crank.
For reference, I use the compact early in the season, and have found that it helps especially on hills that have long sections above 10% grade. Anything else than this and I can spin the 39x23 or higher gearing just fine.