Sorry Mike, should have mentioned I have an older style stronger Cusco center LSD made back in the days. It does not look like your regular Cusco tarmac and I doubt you'll ever find the same one I am using.
It is 4 spider, it is not open, and has clutch pack LSD in it. I get instant power to the rear wheels and do not fry the front wheels like the open center. Car over steers well too. On my car this diff has made a night and day difference and helps with traction tremendously! My car handles more evo like with this diff.
I wasn't aware the Cusco were no longer made so I guess your only option is the Quaife, which as you mentioned can pretty much take out your intermediate shaft.
Either way if I was to choose between a front LSD or a center, I'd go with a center. Like I said I just got rid of my Mitsu clutch pack front LSD as I feel it was no longer needed. With just a front LSD all you are going to do it fry both the front wheels if your making serious power. I'd rather a shift of power to the rear than just a front LSD and open center.
Obviously I am more setup for drag, so I would only really need the front LSD if I were flying around corners which I don't do on the street or drag strip.
So IMO, if you plan on lapping like the 2% of members here, get a front LSD. If your car is more of a street and drag setup, get a center LSD. If your rich and have deep pockets, get all the LSD's. If your feel that your DSM is cheap enough to just get an LSD insert, go for it
Last thing I want to do is recommend an $1800 diff for someone with a DSM worth only $800