I'm new to reef central, hope no one minds this as my first post
One thing other responders haven't touched on is that this lens is going to pull double duty for your wife. The lenses mentioned are all good macro lenses, however there's something to keep in mind.
Virtually all 1:1 true macro lenses are very good optically, so any lens is going to be very much like the next whether its made by Nikon, Canon,Sigma,Tammy.
So what sets them apart?
Price and focal range. The more focal range you want, the higher the sticker.
The difference between a 60mm, 100mm, 150mm, 180mm 1:1 Macro?
Whats known as working distance, or more accurately the lenses
minimum focus distance or MFD.
Why should you think about focal range?
Because you wife wants it to shoot bugs, flowers, nature stuff.
So while a 60mm or 100mm will do perfectly fine for shooting fish in a barrel, if your wife really wants to do nature subjects, the more working distance she has, the better.
To give you an idea. ( I have to guess at 100mm as I don't own it)
A 100mm macro lens will have a MFD of roughly 6-8inches
My 180mm has a MFD of 18 inches.
If you were a bug and very low on the food chain, how would you feel with a great big black eye hovering over you that's 4 inches away? 6 inches away? or 18inches away? Then to, when shooting Macro light is very touch and tricky. The more "working" distance you have, the less likely a shadow being cast by your lens/body will get in the way.
Why did I buy the 180mm (at more $)?
Because I'm a professional, however I shoot for various outdoor/wildlife and nature magazines. More "working" distance. Thats important when shooting insects, butterflies,etc who don't like big scary looking things hovering over them.
I am not saying you "have" to get a longer lens, but if you can save and get it in the budget, your wife will appreciate having the extra working distance.