I have tried both and I actually prefer men's knicks over womens and definately bibs are the way to go. It just comes down to personal perference. I will only wear 2 types, assos and santini. (highly recommend assos) They are a little on the expensive side but if you're planning on riding a few ks, it is worth the extra bit of money to ensure your butt will thank you.
Also for long rides chamois cream is a good idea too.
I love my pearl izumi bib knicks, aside from the fit over hips and chamois, I also love that they do up at the front so the straps don't drift round the boobs!! It is similar to a chest strap on a back pack . <3
Big drawback of bibs over waisted nicks is having to strip off to go to the toilet. Dunno how you girls go on a long ride especially when it is freezing cold. Try waisted ones first and get them fast before your wife is put off cycling forever. If she gets more serious, then give the bibs a go, with the above caveat.
Netti make some good priced ones and you could also trying buying from avantiplus on-line which is having a massive clothing sale ATM.