The main thing which makes bitcoin decentralized is that there's no central authority which can seize your coins. Its the same with Ripple, XRP at a ripple address cannot be seized or moved (without the private key for that address).
Issues of coin distribution and mining versus no mining is a separate concern. From my vantage point, the great majority of LTC belong to Litecoin founders/investors/miners while the great majority of XRP belong to Ripple founders/investors/gateway-partners. There are trade-offs with either distribution scheme, but "coins to miners" is just one scheme (not the defining criteria for decentralization).
Ripple has been pre-mined and is not decentralized, but in fact controlled by one entity (regardless of what the creators may claim).
There is a huge difference here. Not even a comparison.