How different it is to ride with same gear ratio with 26" and 28"
Yes. You can do the math on this: (chainring ÷ sprocket) × wheel size. With a wheel that has an outer diameter at the tire of 26", you'd get 78 gear inches. With a 28" wheel, it would be 84 gear inches, or about 7% higher. 

On a multi-gear setup, I've found that a 7% step is just about the smallest step that's really noticeable.