Get one with wheels!
I'm just back from a week in France which was brilliant, but taking a bike bag on the the Eurostar and TGV was an absolute nightmare(and ended up with me stranded in Bordeaux while my boyfriend and both our bikes merrily travelled on to Paris oblivious to my absence - but that's another story). We got DHB bags from Wiggle which were enormous and already weighed 6k with nothing in them. With the bikes and all of my clothes in it was nearly impossible to carry. If I was taking my steel hardtail instead of the road bike I wouldn't have even been able to pick it up. My first days were spent with enormous bruises on my shoulders from carrying it. I had to resort to pretty much dragging it across the Metro stations in Paris. Although the eurostar and TGV websites say that they take bikes in bags no problem it's not true! We had a few words with guards who were luckily very accomodating. They really have to be large suitcase size and no bigger. The more compact the better as the TGV certainly didn't have a baggage car. Also if you can get one from a bike shop then do, buying online was tricky as the dimensions looked fine but then you get it to realise it's very different to expected.
It was a bit silly of us to think that there wouldn't be a huge difference between the various types of bag but at least we've learned from experience. I'd definitely ask around and get plenty of recommendations for the next one.
And after all of that we only managed one decent ride because it was far too hot to do much. Luckily there were plenty of vineyards to explore...
