The family and friends of mine that have used easyjet and flybe have given them ok reviews, so long as you have realistic expectations - ie you get a flight....and nothing else.....boarding is a scrum, customer service not always fantastic, but for what you pay they are ok

I don't know anybody that has flown Ryanair so can only go on reports I've read which have been decidedly ropey...

As for ferries, they tend to be relatively reliable - only foul weather tends to cause cancellations,, and in late March the chances are you will be fine.

The other consideration is that if you are in say Glasgow you will have to drive a distance to the ferry port however there are some nice scenic drives up in that neck of the woods.

Personally if it were me, I would be tempted to take the ferry.