Being sister ships, they are almost identical. The only things you'll find different are a couple of specialty things, like the small restaurant in the Boardwalk area.
The only thing really different is that the Allure is one year newer and is 5cm bigger! That's about 3" and while it doesn't sound like much, when you're talking about a ship that weighs 220,000 gross tons, that equates to a bit more weight.
So it really doesn't matter which one you go on; Oasis goes out on Saturdays, Allure goes out on Sundays. One will do the Eastern Caribbean one week, while the other does the Western Caribbean, then the next week they swap itineraries, but the ports are the same for both.
Personally, I'd just choose the one that gives you the best deal, just make sure you use an agent as they'll give you better service and help you more should you have any problems.