I own a HFT 500 and have shot both the Goldstar and Hammerli AR20 rifles.
First of all, I absolutely love my HFT 500, especially now that I have got the trigger sorted to an acceptable level. It is extremely accurate and very consistent. The stock gives plenty of adjustability and with the addition of a hamster (I have a home made one on mine), it can be made into a very comfortable rifle to shoot. The FAC air cylinder also gives a really good shot count, although mine does seem to prefer being filled to around 160/165 bar and shoot down to 110/100 bar.
The Goldstar is also an excellent choice for HFT. The stock has a lot of adjustment and it does seem to be a very consistent rifle. A club mate has recently bought one and he is returning some very high scores with it. As it is still a relatively new rifle, only time will tell if there are any inherent problems to look out for.
The AR20 is a great target rifle considering that it has been built to a price. It offers everything that the target shooter will need and is amazingly accurate. The only downside is that having been built to a price, they maybe will not offer the same long term reliability, or hold their second hand values as well as the Air Arms or BSA options.
Hope that may help.
Founder member ~ Fort Airgun Club
Air Arms HFT500, Sightron SIII 10x42 MMD
Air Arms Ultimate Sporter, Sightron SIII 10x42 MMD