make a trip to your local apple reseller and ask your wife to have a grip on both devices and choose. 

What phone is your wife using now? If she's already using big screen phone, then go for 6 Plus. If she's using smaller screen, she may not get used to big screen phone. My best friend was using 4s and she can't accept 6 Plus size, so in the end bought 6 only.

then i would suggest the 6. if she uses her phoen with 1 hand usage most of the time 6 would be just right for her... also for cases wise 6 has more choices too. 

You can try your luck with resellers but I'd say online is faster

Apple store, warranty claim is good and it's fast. The time you source for an iPhone 6 in any reseller is wiser to be spent on few mouse clicks. If you buy it today, most likely you will get by next Wednesday.
Most of the resellers still out of stock anyway.