I'm assuming you're asking about these two separate scenarios:
- The wife tells the husband: "You wouldn't be spending it for fun."
- The Wife tells the husband: "You wouldn't spend it for fun."
In both cases, assuming that they're both supposed to be statements, not questions, the only real difference is the tone in which the wife is speaking to her husband and a slight difference in when/how the "spending" of the money is taking place.
The first case, using "be spending" could, depending on the context of the rest of the interaction, be read in a more relaxed/gentler manner by the wife. The use of "be spending" also dictates that this action will continue to occur in the future, that it won't be a one-timed fixed event, but a continual spending on the part of the husband.
The second case, using "wouldn't spend it" reads as a much more direct statement by the wife. In the limited context you've provided, the wife seems more authoritative and demanding, essentially telling the husband where he would spend the money/not spend the money. In this case (once again slightly) there's an assumption that where/how the money will be spent is already decided/will be a one-time fixed expense in the future (the husband will spend it once, not continually)
I'd say the biggest decision here is tone, but you might also wish to ask yourself if you're intending for the action to be continuous or not.
You might be interested in reading about the Future Continuous tense, as this is technically what you're doing: