I'm not sure that 27 ---> 28 would make enough of a difference. I'd lean towards either the MTB rear derailleur and cassette, or a triple crankset (depending on the shifter model).

This all depends on how much you need to lower the gearing. Is there any possibility of borrowing a bike with either configuration to test out?

Also, any time I'm experimenting with gearing, I play with a gear calculator to help me "visualize" the change.

Good luck!