Do not compare the siacoin with Ripple because Ripple is one of the best finance coins so far besides Bitcoin and Siacoin it is a coin for data storage.
These two coins have different platforms and newborn Siacoin in 2015 while Ripple is from 2012.
Ripple is a transitional project at best. Not about true decentralization where I believe true decentralization will one day be possible.
Siacoin is completely different with a great purpose.