I wouldn't worry about what makes sense on paper, or what other people might be using.. If you feel like your having trouble getting up climbs with proper cadence -- i.e., if you have more than a couple climbs on your normal ride where your in the 25t, and really pushing it and hurting.. Then by all means do the 11-28.. You need it to make your biking more enjoyable, which will make you a better and stronger rider all on its own accord. You will also be less likely to suffer knee issues, which is always nice.. and really, your legs will be fresher throughout your entire ride. Weight between the two cassettes is a non-issue. Performancebike.com has sweet deals on shimano cassettes.. Ultegra is like $65 or $70.
Now, for the real issue.. If your running compact, and need a 28t rear cog, its obvious your bike fitness has ample room to grow! This is good news, not bad. Focus now on just being as comfortable and happy as you can be when you ride, meaning the 28t cog if necessary. The more you ride, the easier it gets for sure.. even if your relying on that 28t cog. In the off season, and right now, you should be switching it up w/ some cross training, and quad building exercises.
I have been running a standard crank, with a 12-25t in the rear for ever.. Recently, I finally switched to a 12-27t rear, as there are some big climbs in my area, and I was sick of pushing so hard in my 25t cog. Made ALL the difference for me.. Overall, I'm much faster on those rides, as my legs recover much quicker after the couple climbs. I RARELY use the 27t, mostly using the 12-24 range, which works for me. But i'm SO happy that 27t is there when I need it.