I own and use both a Fujifilm GF670 and a Yashica Mat-124G TLR. I am the original owner of both cameras.
In my opinion the biggest handling difference between the two is that with a TLR you can put your hands all over the camera without having to watch out for anything other than the lens. With the GF670 there is the constant need to be careful where you put your hands because of the exposed bellows.
If you are already comfortable using a bellows style camera (many of my cameras have a bellows, so care in handling already comes second nature to me) then you won't notice anything much different. If not, it may take a period of time to adjust and become accustomed to being a bit more careful.
Another moderate difference is the focusing regime. TLRs have those nice big fat knobs on their sides that are easy to locate, grasp, and micro-adjust. The GF670's focus ring is further away out on the extended lens. For me I must cradle the camera in my left hand in such a way as to not crinkle the bellows. Then from this position the most natural finger to fall onto the focusing ring is my left thumb.
It took a while for me to become comfortable focusing with my left thumb only. Using any other additional finger and I must then support the weight of the camera entirely by grasping it with my right hand, and this is not always a steady configuration. I should also mention that I have very large hands. (I can pick up and hold a basketball with one hand.) I'm not sure that if my left hand were any smaller that I could effectively support the camera, not touch the bellows, and thumb-focus, all at once.
I love the GF670 and wouldn't trade it for the world. But I would suggest that if you can find a way to do so, it would be to your benefit to handle a GF670 before you buy one. Just to know a little better what you would be getting into before spending your money. It is a bit of a different beast.
Best of luck in your decision.