Difference in these inks?
 

I saw a video using Momento Opaque White ink on a window sheet to create a "frosted window" look.

I don't have this particular ink but wondering if I could use Stampin Up's white craft ink?

They are two different types of ink. The Memento, being a hybrid pigment ink, will dry on window sheets. Stampin' Up's is a craft pigment ink, and wouldn't dry in your lifetime, unfortunately. You could try to heat emboss, but only if your window sheets are thermal sheets, intended to run through laser printers. Do you happen to have white Staz-on ink? That would work.

Thanks. no I don't have a white Stazon. Guess I'll put it on my shopping list. :D

White StazOn is awful. If you do buy it, and I wouldn't recommend it personally, make sure to buy the reinker to because you will need it every single time you use the pad. In my experience, Brilliance inks from Tsukineko will set on acetate...just give it plenty of time.

You can't see much of it, but I used Brilliance on the acetate layer on this card: http:////www.splitcoaststampers.com/.../photo/2746009 stamping before work and finishing the card later.

Brilliance is another great one. I've never had trouble with the Staz-On, though. It is a finer-bodied ink that's designed to dry very quickly. Do places still sell the pads without the reinkers? I wouldn't recommend using it on paper, though, that really does look awful!

I am going to second Cook22 on the white Staz-on. It is, IMO, the worst inkpad ever. Horrible, sticky ink. And as she says, you need to reink every single time you use it. I got sucked in by the Staz-On Opaque idea when they came out, bought the white, the Buttercream, the pale pink & Pale blue. They are all awful- always needing re-inking, I can see why they sold them in a pack with a re-inker!- and I have actually had the inner lid on the pink become stuck to the pad as it dried out.
I'll also second MSBetsyZ comment that it's terrible on cardstock too. Even when heat embossed on dark cardstock, it doesn't look white.
Also seconding the Brilliance Moonlight White, it works great on cardstock and acetate, so a good allrounder. Definitely helps if you heat set it though.
Helpful information on all Tsukineko inkpads(which include Staz-on & Brilliance, as well as a host of other commonly used pads) here.
Although they say it's fast drying, so difficult to heat emboss on cardstock, I've never found that to be the case.