Leigh:
I pondered this for quite some time before making my choice in my '87 944 ... the white one that caught a few spec-cars in it's day.
I think for your needs and the amount of small track/AX driving you do you'd be better off in a GT-Torque-Biasing differencial. As a 944 owner, I have direct experience with this (sorry Kary, Kris, Martin .... but you guys are in 911's, and there are some significant differences from 911 vs. 944 with respect to LSD/TBD set-ups).
My reason's for TBD from Guards Trans.:
1) Bullet-proof quality build and function. No worries about fragility.
2) No need to remove/replace diff. clutches. Once the tranny is dropped and the TBD is installed, you're done with it. Poke the tranny back in, and go drive.
3) With all the tight-turning under load at AX's, the stress on the Limited Slip Diff. Clutches at AX's may be a little higher than Big-track driving ... thus a slim possibility of needing to R&R the trans. more often for maintenance (Check with the shops to get opinions on this. It could be crucial to 944 maintenance costs). With the TBD ... just drive it.
(Note: if you find yourself leaning toward more Big-track events, I agree with Kary: an LSD may be a better choice because you can "order" the different excell/decell variables, you will have greater stability braking ... esp. trail-braking at high speeds, and you won't be stressing the clutches quite as much on the "more open" corners of a big track.)
4) The TBD doesn't affect the way the car slows under hard braking like an LSD. For some that is good. For me, I always liked a "free" diff under braking. But that's just my personal driving style. Everyone's style is different. I found with a more neutrally balanced set-up, the 944 doesn't appear to have the same "tail-wiggle" under heavy braking as the 911's ... but that is just my observations having driven both the 944 and the 911 on big and small tracks alike. Others may disagree with me.
Martin is spot-on about the heating issue and TBD's. Since you have more gears turning inside, the heat really builds up quickly. I ended up using a mixed-bag of trans. lubes to keep things cool, and functional (Royal Purple/Swepco/Liqui-Moly blended from a viscosity range of 75 to 140). I also found a used 944S trans. that had beefier final-drive bearings for handling the extra heat and torque stresses.
You will experience much better exit speed on cornering with either set-up. Again, it'll boil down to personal driving styles, personal preferences, money, and vehicle use.
Good luck with it. feel free to PM me if you want more details on my old set-up.
Happy New Year!