If you go iPhone, they have iMessage for texting which costs $0 on pre-paid but you both need to have iPhones. Use WiFi where you can (home, work) and find a pre-paid option for other times. iMessage is handy if a lot of her friends have iPhones.
If her usage pattern doesn't involve lots of streaming movies or streaming audio then you really don't need a costly plan.
My wife is non-technical and the iPhone is wife approved. Her mom (in her sixties) just got an iPhone and she can use it too.
Serious answer. As suggested why not let your wife decide what would fit her needs best?
For someone that likes to tinker, or has a lower income I'd suggest Android. Android is more flexible to customisation + the phones are at a significantly lower price point either saving you money overall or allowing for more frequent phone updates.
I currently run an Android as my personal phone then tend to run a second ever changing OS on my work phone out of general tech interest. Currently I'm on Windows as the second phone but am getting Sailfish and Ubuntu curious so I feel an upgrade coming. I wouldn't recommend the Windows phone from an experience POV (though prices can be great). It's maybe 80% as good as iPhone and still needs polishing. Windows 10 might change this as the phone will become your PC on docking which I believe will be an increasingly popular category for phones.
As most said, unless it's a surprise gift, let her decide.
If it is a surprise, try to get her feedback in more subtle ways (I'm pretty bad at that, but I guess asking about her girl friend's phones or something).
In general, it depends on your money availability, general use case, IT ecosystem at your house or where she'll be using it, etc.
If money is no objection, she won't be using it for programming-related stuff or obscure hobbies, can afford apps, etc. or you have a heavy Apple ecosystem, then the iPhone is probably the best bet.
If money is a concern, then it can be either an Android-based system or Windows Phone.
For the very low-end smartphones, I recommend Windows Phone, some Android smartphones are terribly underpowered and deliver an awful user experience.
Personally, I have an Android device because it's the one that fits my needs the most (but I do use it for programming), and my country is 90% android smartphones so I'll have every app available.
That said, the reason I'll almost certainly stay on Android is access to all the programming tools: J, Ruby, Python, Forth, Octave, etc. It's just too nerdtastic even though I think TouchDevelop is one of the coolest things ever.
My teenage son was playing rugby and while the game was going on the changing room was robbed.
The only thing the robbers took were phones. Interesting, only iphones were taken and the only phones left were a badly cracked iphone and my son's Nokia Windows phone.
What made my son particularly happy was that, the reason he has a Nokia is that he had his iphone stolen while on holiday and didn't have enough to buy a new one.
So, if you have a habit of leaving your phone around, the conclusion is to either smash it or buy a Windows phone!
I can't find a newer analysis, but probably worth pointing out that they mention manufacturer security improvements reducing thefts, and iOS has obviously had some of those since 2014.
[0] https://www.gov.uk/government/uploads/system/uploads/attachm...
its just personal choice in the end
But if I must answer: iPhones seem to be extremely popular amongst women in particular (if you look at the demographics, women like iPhones, Samsung Android phones, and then everything else is a distant third).
That can be a drag if you like having the latest and greatest apps. Other than that, Android is awesome.