Praslin is an easy ferry ride from Mahe--just over one hour if I recall correctly. You then get another ferry from Pralin to La Digue--I believe it's just under one hour.

Prices for the ferries are reasonable--certanly much cheaper than flying.

It's DEFINITELY worth visiting both outlying islands.

Challenge is that accomodations on these islands is either very expensive luxury or less expensive and basic.

We did an overnight in each--and that was about right. Although it may be more judicious to spend one night on Praslin and two nights on La Digue--the beaches on the latter are among the most beautiful in the world.