just had to decide the same myself, ppg straight cut is what i went for, but mine is a weekend toy and i love the whine from it
i had exactly the same choice to make about 5 months ago.
replace with a standard/rebuilt box £600 ish, not good if your going for more power
6 speed £1800+ strong, but some people don't like the ratio change
5 speed helical or striaght cut ppg box £2700 strong as you can get
6 speed helical or straight cut ppg box £4700 strong as you can get
finaly option is prob nearer 8-10k and thats a full dog box,
PPG gear set and dog gear engagement system, its not cheap, strongest type of box you can get, but you have to double clutch
the PPG kits would use my existing 5th gear for the top gear to keep it nice and quiet for cruising.
in the end i went for the the ppg straight cut box, and have to confess i love it, the sound when you welly it is fantastic, i've got to the stage where i some times don't even notice it.
heres a little video of it taken from out side, mines the blue classic early on, but you can tell from the noise as i drive past then leave the road
Video of Scooby crescent - Photobucket - Video and Image Hosting