Generally, the larger the ship, the more stable it is. Sailing from Southampton has the benefit of avoiding the misery of airports and you can take as much luggage as you want. The only disadvantage is that if you want hot weather, it takes a day or two to get there. However we quite enjoy days at sea, where you can relax and get to know the ship.
Sailing in a big ship, if you suffer from sea-sickness, is certainly preferential to sailing on a small ship out of Southampton because the Channel, Biscay and the Atlantic or the North Sea can and do often have big lumps in them. Don't forget that you can always wear wrist bands or take pills or have an injection (these are in order of effectiveness) if you have that problem. Going out from Southampton is often a little cheaper than flying somewhere but not necessarily so. Leaving from Southampton invariably means quite some time before the really interesting ports begin. Flying out of the UK gives you far more places to visit and cruise in if your time is limited but it does mean that your luggage is restricted and flying can be a bind. You can also be sick on a plane, it's just that you are on a plane for less time than a ship. I suppose what I am saying is it's horse for courses and each has its own advantage and disadvantage....you pays ya money you takes ya cruise....Neil Dow.
I would definately fly out because you waste so much time sailing from southampton where as if you fly you can just get started and have a port really soon. I think it takes around four days usually going from southampton to your first port - i really prefer to fly.
A larger ship is better for stability. If you do suffer, I found the sea sick pills they give you on board are brilliant and work very quickly. We always sail from Southampton and really enjoy the 2 days at sea finding our way round and using facilities on board. We don't enjoy the airport hassle - boarding at Southampton is quick, easy and very straighforward. Hope you enjoy whichever you decide on.
The Bay of Biscay can be awful if you suffer from sea sickness. Probably better to fly and then get the ship.