magic item feature comparable to magic resistance?

[foobar1969]

The 20th level Wizard in my game will soon capture a Robe of the Archmagi, or a variant thereof. I'll leave the AC & DC features as-is, but I'd like to replace the magic resistance with some other boost of equivalent value. Which of these do you feel are equal to "you have advantage on saving throws against spells and other magical effects"?

• 5E stat booster: Your [ability score] becomes 19.
• 3E stat booster: Your [ability score] increases by 4? by 6?
• proficiency: a saving throw? two saving throws? a skill and a saving throw?
• advantage: a saving throw? two saving throws? an ability check? all d20 rolls for an ability score?
• short rest spell slot: 3rd level? 5th level?

[Segev]

In order to figure out what a good ability would be, first I would like to know why you don't want magic resistance on it. Is it a balance issue (you think it's too good)? An overlap issue (the wizard already has that)? A flavor issue (it wouldn't have it for reasons of what it is)?

Granting proficiency to Dex and Con saves is probably the most equivalent thing I can think of, but I won't say that it's the best option.