The DFF3 offers a heaving compensation feature and is more comparable to the FCV1150 by design. If you are looking for the best of both worlds (black box and standalone) you might take a look at the FCV1150. The FCV1150 is the only standalone sounder that also has a network port allowing it also to work on the NavNet3D/NavNet TZtouch network (like a network sounder).
CHIRP sounders require a CHIRP transducer, so if you expect to upgrade later to CHIRP, you should buy a CHIRP transducer for your new install. The CHIRP transducers will work on non-CHIRP sounders but not the other way around. CHIRP would require new hardware so don't look for any software updates to provide CHIRP in the DFF3. It has taken CHIRP to get others even close to what Furuno has been doing for years. When we add the feature, you can be darn sure it will be worth having.
When it comes to comparing the FCV295 and the DFF3/FCV1150 you might poke your head in the sounder/fish finder thread. There is a lot of good info there. Snips is a wealth of knowledge.