Pay monthly Catalogues have many benefits, one of those is being able to pay slightly more each month allowing you to reduce the interest you pay.
If you are in a position to do so, making an additional payments even a few pounds can reduce the overall interest you will pay.
Some Catalogues can charge in excess of 50% APR, so keeping interest down as much as you can will always save you money. Most Catalogues work like Credit Cards, requiring a small percentage to be repaid each month, Paying over this amount will result in less interest being charged.