I had trouble choosing between In A Snap and Polishield. I gathered from the producer's description that they did exactly the same. So what do I usually do when I can't decide? Buy both! So that's how I came into possession of these two top coats.
I've put them to several tests and I must say there is no difference between them (I didn't read the list of ingredients, though). But what needs to be said is that I don't regret buying them both cause I've got two great top coats which will last longer.
Both of them speed up polish drying time, give shiny finish, prevent chipping and neither of them makes polish shrink. What else to ask for?
EDIT: After finishing the bottle of Polishield I switched to using In a Snap and I managed to figure out the difference. In a Snap takes a bit shorter to dry, but it tends to shrink polish, not as much as Seche Vite, but it does, whereas Polishield doesn't.
Disclaimer: I bought the product myself for my personal use and I am not affiliated with any company. I am not paid to do this review and everything I said here is my genuine opinion.