Hi Tony, my suggestion would be to hold out for a mk1 Hw50 but if you already have one be happy with what you've got.
The Hw99 is overrated, I bought one when they first came out, was unhappy with it and it stayed in my gun cab for over 10 years in parts.
All Hw need to do with the 99 is fit a one piece cocking link but until them, it is a problematic rifle.