You will want to plug up the holes if you want to use the canister. Its more tricky using the overflow holes for the canister, but its do-able (according to the people that used it and says it works). I think those people also have a lot of knowledge about utilizing an overflow with sump. If you're into plants, then you definitely want to use a canister (for C02). If you think you can live with low-light plants only, then it might want to look into utilizing the overflow and a sump. If you just want an easy setup and not have to deal with the dozen hours of research about sumps, then please go with the canister.