But this thread, and MN in general, seems to indicate that people are more weirdly snobby about prosecco.
I think cava is supposed to be 'better' because it's made by the same method as champagne, whereas prosecco (I think) isn't bottled until after it's second fermentation, but tbh, I can't tell a great deal of difference. As it happens I had some Morrisons 'The Best' prosecco over the weekend and that was very nice.
But I generally go for cava because I like all things Spanish, but that's just me and I'd never turn my nose up at any type of fizz, whether it is £5 Aldi prosecco or vintage champagne.
If you want it to be a nice treat and more acceptable to those who like to think they're more discerning, I'd spend a bit more than the minimum, eg £8 a bottle, but look out for offers, like you can often get 25% off if you buy 6 bottles, that sort of deal.
But in terms of whether cava or prosecco would be preferred, you're literally flipping a coin, but I agree with a pp that a Cremant, being fizzy wine made in France but not in the Champagne region, is generally more highly regarded than either.