I own both, as does jsherman. Both lenses are excellent and the focal length is different enough for you to use both, if you are able to afford them.
The Sigma70 is my favourite lens by far. It renders pictures like there's no tomorrow, the bokeh is smooth, it focuses fast for a macro, it never misses metering the light correctly, it's awesome for portraits, it macros at 0.91:1, it's wowzers sharp (!), it's almost impossible to get it to PFringe, MF is awesome, it's sexy, it's uber useful for many many situations... It's also slightly heavy and fairly large in size for it's focal length.
The FA77 is a totally different beast. It renders pictures with a wonderful subtle 3D effect, the bokeh is smooth and beautiful, it focuses quickly, metering can be touch and go (I have to chimp a couple of times to see my exposure is right), it was made for portraits, it's sexier than...well, use your imagination, it's small and discrete, it's MF is awesome, it's also useful for many situations. It's also very prone to PFringing.
I have both and will keep both because they render differently, and because I make a side living out of taking pictures of people (and see the need for both lenses). If I had to choose between the two I would be very hard pressed to pick either...so I'm no help there.
I guarantee you will love it if you go for it. Ultimately you will have to decide if the extra couple of stops of light are worth it, whether the size/weight difference would be useful, and whether the 7mm focal length difference matters (it's noticable, but not extravagantly different).
I love both of them...so, good luck with your decision
c[_]