I bought my electronics in June; I went with the Garmin 240 because of space considerations at my helm. I've heard nothing but great things about the Furuno 6100 from others that use them. For my needs, the Garmin 240 made sense. It is a great unit and I can recommend it without reservation, but I am not nearly as knowledgeable about fishfinders (or fishing!) as others are here, so I guess it really depends on your needs, your space, and your wallet. I put the money that I saved by going with the 240 Blue towards a better GPS (Garmin 182 color); the GPS was more important to me. At the same time, I do not feel that I sacrificed the quality of the fishfinder relative to what I need it to do for me. It's very user friendly and easy to learn on, which is what I'm doing now. But if you would benefit from the extra features of the Furuno, and are an avid, diehard fisherman, maybe the Furuno would be a better investment for you. Whatever you decide, good luck!
P.S. Have you gone out and played with the units yet? Definitely go take a look at them, and play a bit, see how they feel.