I think it's easier to think in money terms, rather than percentage terms.
First compare AIB to KBC
So if you switch to KBC, you will get €1,850 cash into your hand plus a saving of €250 a year.
The next question is which would you think will be cheaper in the long-term?
KBC has been cheaper than AIB for new customers for some time. But for a long time, they exploited existing customers by refusing to allow them to move to the new customer rate. While AIB has been more expensive, they have been fairer to existing customers. They pass on rate cuts automatically to those on variable rates.
It's hard to know. Overall, I think that KBC is likely to be cheaper in the long-term, so it's probably worth switching.
Next compare KBC to Avant
You will save €500 a year, but you will be €3,000 a year worse off up-front.
So it would take you 6 years to recover the difference.
So KBC is better than Avant as well.